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.
depending on what worries you, he may be fine. vaccines *may* remain in the system longer than their recommended length of time. i’d get him to the vet to make sure he’s OK and maybe discuss titers with your vet.
A vet check would be good..Even indoor cats should be vaccinated for minimum rabies vaccine–for cases like this when “indoor” cats escape, it is important to have rabies up to date.
Your vet can pull blood and get titers checked to see if they are ok w/vaccines if you don’t want to re vaccinate
good luck!!
~kelly