I Have A Cat That Is 15 Yrs Old And Has Been Fed IMAS Dry His Whole Life. 5 Mouths Ago He Lost All Weight From 10lbs To 5lbs. We Now Feed Him FRISKIES Wet Food But Is Still Throwing Up, Why?

We've had him as a kitten (a junk yard cat). He has always been very needy, but after we got him feeling better he wants to eat every 2 hrs and is still throwing up two to three times a week .His fur was matted and he was disorented (staring at the wall and crying). He's still thin and his coat is better and acks like himself most of time. I want to know if he's just getting old or is there something i can do to stop him throwing up. Different food? More brushing? Deal with it?