My son who is now 16 no longer plays with his Lego sets. He would like to preserve some of his larger sets like Jabba the Hutts sail barge. Is there anything we can spray or paint on to hold them together? Or do we need to take it apart and glue it?
Although it can be done (I will share below), I would sincerely discoure you from gluing your LEGO sets. You will very much regret it later. Here are the reasons why: LEGO is meant to be a building toy - put together and taken apart many times. Once you glue a set it completely looses its usefulness and becomes nothing but another item to take up the shelf and collect dust.
LEGO has generational value. Your son may have lost interest now (very common in teen years), but may want to get back to the hobby later. He will definitely regret gluing his sets. Also, even if he doesn't get back to the hobby, if he will decide to have kids in the future, they will greatly appreciate having the opportunity to play and build with LEGO. Many LEGO fans inharited their LEGO sets from their parents and even grandparents. The long-term value of LEGO is truly amazing.
And this brings up the third point against gluing LEGO. The sets you mention have significant value that will only rise when the sets get retired. If your family ever gets into financial trouble and need to get some cash quickly, you can sell those LEGO sets - but not if they are glued.
I would encourage you to consider just boxing the sets (either built or taken apart) and just store them away carefully. It is really your best option that would keep both the value and the usefulness of the sets.
Having said that; if you still want to glue the sets, yes, you will need to take them apart and glue them. The most gentle glue that would not damage the LEGO elements too much but will hold the model together is simple white school-glue. You only need to add very little in between elements so the glue doesn't come out at the edges.
Another option is to just wrap the whole set in thin plastic sheet (the kind used for wrapping food). This is more for storage and moving the set around, but the tight-fitting plastic prevents the set from falling apart.
Use MEK (Methyl ethyl ketone). Can buy it in hardware or auto body shop supplies. In auto body work it is used to degrease and clean adhesives off surfaces. It is also a major component of ABS plastic cement, which you can also use but it's messier. You can also put a bit of MEK in a glass jar and then throw in old lego pieces to create a filler and cementer. But that is a rare thing to need.
I use an eye dropper bottle (glass of course) or glass syringe to glue parts together. You MUST be in a well ventilated area, and I mean well ventilated. MEK melts plastic so put the pieces together before it evaporates. If your eyes are near MEK they will be bloodshot, so have a fan or do this outside, like I do. Also wear those cool blue non latex gloves to protect your hands.
I use MEK to glue major structures together that fall apart - like supports. In tall structures I will glue the structure into two halves so that they can be stored partially unassembled, and then put back together. It is also useful to glue things like space shuttle parts that continually fall off when played with. I never glue all of the pieces, just the major parts that support the toy, that way it can still be disassembled.
MEK works great and I love how it keeps builds together that would otherwise be lost forever in masses of parts stored in bins and tubs.
I have Lego sets that have been on display for years, including the relatively fragile AT-AT from 2003. It has stood, fully assembled and almost never touched, since I first got it in late 2003/early 2004. In all that time it's gotten dusty, so I've used various things to clean it: a vacuum with a brush (if you're sure there are no loose parts), a can of compressed air, and a sink of soapy water. You can submerge this whole model, or take parts of it apart, and wash the dust off, in a few minutes.
At the end of the day, it is still in near-perfect condition, because it's never played with and just sits there. I never, ever, wish it had been glued. Why would it need gluing? I can always put it back together if anything falls off. The lego pieces hold themselves together just fine.
There are two areas where its preservation is lacking: First and foremost, sunlight. Sunlight changes the colour of the bricks. I keep this one out of direct sun and it hasn't yellowed too much. But the light-grey pieces are particularly prone to discolouration and I'm sure the outer shell of the model has changed colour somewhat. The only thing that would have helped is UV-protection.
Second, some of the technic pins that hold it together have weakened over the years. These can be easily replaced if they become a problem.
Otherwise, this model has stood as a display piece for 10 years now, but if I wanted to, I could disassemble it and use all its pieces in a new creation.
I offer this as evidence that you do not need to glue models, and as TheBrickBlogger wrote, while it's possible, I would discourage it. It doesn't "preserve" the model in any useful sense, except keeping it together so that no pieces get lost or missing. If the model isn't going to be moved around a lot, or played with, nothing will fall off. And if something does fall off, it can easily be put back.