@Gazz99
No, no, no, the whole point of getting the am4 5700x3d, is that your son already has an AM4 motherboard, literally all you have to do is update the BIOS to the most recent, so it can definately take the 5700x3d, and just drop the cpu in there, job done!
Don't worry about ram mate, the 32GB he has is fine for the games that he played, I only had 16GB, and made it to the top slot in PUBG (survivor, )season 5, if you ever find that you need more ram, because of modding a game or what not, just buy a 64GB kit then, x2 32 GB ddr4 matched kit, save the money you would have spent on the new motherboard, and put that towards more ram later or a better gpu the now