IBD Maybe?

Jake is 14 or 15 years old.  He lives with us on a boat.  He gets brushed well each day.  We are in Indonesia where we have access only to a vet who seems somewhat unfamiliar with cats.  Jake has been occasionally vomiting and seems to have become less tolerant of kibble.  In fact, he has stopped eating kibble now.

He is vomiting occasionally; not associated with eating but with using his litter box (pain?).   He tolerates milk well and is also drinking water so he is not dehydrated. He had been good about petroleum jelly "treats" but lately he's been rejecting them.

Starting about a week ago, he was completely off  his food (but drinking well).  His temperature was normal, his eyes bright.  The vet gave him hematopan B12 and this did restore his appetite for fresh (cooked) fish and white meat chicken.  He also ate a soup of fish/chicken/fish&flax oil/yeast & water.  (We cooked this to kill the yeast so they wouldn't ferment.)

He is still eating still this morning - fish and the "soup" but vomited after using the litter box (before eating)  There are no feces we can find .

Thoughts?  Suggestions?  

Antibiotics?  Enema?  Laxatives?

We do not have access to diagnostic testing right now.  

Is he noticeably straining in the litter box and nothing happens? How long since his last bowel movement. Many cats with constipation will vomit after an unsuccessful or even a successful trip to the letterbox.

Hadn't noticed straining.  Last bm was yesterday.  He just started eating again yesterday (after 3 days without substantial food) so we expect one soon.  His stools had been "squashed", not round, and contain hair.  We try to brush him daily but hairballs remain a problem. Thanks for your feedback.