It's not anyone's place to tell you what you should or shouldn't allow your son to play with. So if this is your family's choice, then here's the best advice i can give you for what you are asking for..
is to NOT make an issue of this. I don't mean ignore it. And just minimize the time u have with the in-laws. What choice do u have,right?
But at 3.5, kids are like sponges. Don't discuss guns..good and bad, do's and don't about it. Just spend time playing 1-on-1 with him with various NEW activities with him..something new that he hasn't tried. With kids, it's all in the novelty and how excited you are about something. You have to be there to play with him..not just introducing and then have him just playing by himself. He won't know the potential of this NEW interest/toy/activity...if you're not showing him all the possibilities that it offers.
Gradually,day by day...get him more interested in which ever activity he becomes more drawn to. Incorporate throughout the day, by toys..drawings, games..pretend play.. videos/movies...based around THAT interest. He will eventually fine a new interest. Kids at thisage are easy to influence. He can't only just like guns (maybe just for now..because it's novel).
My 3.5 year old has always liked things because it was introduced to her by us. Every year, her birthday was a new theme because every six months she got attached to a particular thing. First it was elmo, then princesses, then fancy nancy, and now American girl.
p.s. just want to say that don't make a big deal of NOT DISCUSSING it also. Just be nonchalant about it..and move on to the next topic as quickly as possible.
i have a son, and he's already showing signs of fighting and battles because his dad showed him spiderman, ironman...a bunch of 'other man':)..i don't mind. so to each it's own