Senior Cat Weight Issue

I have written in the past about my senior cat, Simba. I've only had him since March, but it seems like I've had him longer.

Shortly after I adopted him, I took him to the vet for the amount of times he would vomit in a day. He was diagnosed with IBD and given a treatment. His puking has decreased significantly from about 12-15 times/day to 5-8 times/week. However, over the past few months, his weight has dropped a great deal. Since I've had him, he hasn't been a heavy cat, but his weight fluctuates between 7.5lbs to 9lbs. Recently, when I look at him, he just looks so skinny and feeble. He seems to be eating and drinking normally. On top of the throwing up, Simba seems to always be constipated; he can eventually get it out, but it's hard as a brick (and usually never in the litter box). The vets (I've taken him to three different vets) have tried everything to remedy this, but nothing seems to "cure" it.

I guess my questions from this convoluted mess are: 1) What can I do to help increase Simba's weight? and 2) What can I do to possibly relieve his constipation?

Simba only eats dry food at the moment. I've given him wet food before, but he throws it up immediately after he eats it (when he decides he wants it). He eats the same thing my other two cats eat, Nutro Grain Free Potato and Duck. I was giving him IBD specific food (Hill's Prescription Diet i/d), but it seemed to make his constipation worse. 

When I adopted him, the humane society I got him from said he was about 10 1/2 years old, though I believe he's older; none of the vets I took him to could tell for sure.

Any help or insight would be appreciated.

You might try an old trick used by folks rearing kittens on bottles.  Constipation can be a real issue for them, too.  A few drops of olive oil added to the food, although with him only eating kibbles that might take some creative dosing.  Or a syringe with a bit of olive oil shot down his bitty gullet (that's how I have to give Hekitty her flea meds).  No fun, but it might help.

If he's a kibble kitty, there is always the chance that he's not taking in enough water.  That's why canned or raw foods are considered preferable.  But if he gets sick on them, he does.  Hekitty is a kibble kitty, but she also sucks back water like a camel in August.  I am continually worried about diabetes, but her blood work is all normal.

first of all, talk to your vet.  our cat has IBS, and he was throwing up all the time.  the vet gave him prednisolone (we have it compounded into tuna flavored liquid), and that once-a-day dose has got HK's vomiting down from multiple times per day to once every 5 to 7 days.

our cat is on 1/8th of a teaspoon of miralax powder twice a day for his constipation.  i melt it in a tiny bit of water and then mix it with some cat foot gravy, which he thinks is quite a treat.  that wouldn't have worked before the prednisolone, but now he that he doesn't vomit much, he LOVES having gravy from fancy feast gravy lovers or grilled flavors night and morning.