Is there a way to glue already built sets?
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.