this might sound like a copout answer, but you really need to ask the vet about this. i would be worried about the weight loss, yes, but a bulging stomach would have me very concerned.
if your vet is worth their license, they’ll be frank with you. please, open a dialog with them.
first of all, i’m so sorry for your losses and all of your upheaval. that’s a lot to deal with all at once. i hope you’re doing okay.
secondly, he may just need reacclimation to your home. please try to be patient with him. my male cat was this way when i brought him home from the shelter…he was absolutely NUTS for a while. the only thing that really helped was increased interactive playtime, so i bought a wand toy and went “fishing.” that seemed to help.