Best food for underweight cat

Ok, so Parker has been to the vet recently (had surgery) and the vet said his weight is good at 9lbs but to me, he feels too bony. I know I should be able to feel his bones to some extent but I hardly feel anything between the skin and bones. He isn't like, emaciated or anything but definitely too small. I think he would be much healthier feeling at about 11lbs. He is 10 years old now.

So my question is, what should I be feeding him and how much to get him to get some meat on his bones? He is on various brands of wet food now (bouncing between high and low quality since he gets bored of food quickly) and eats two meals a day. Sometimes three if he is interested. 

 in advance!

Well....you want to look for a high calorie food if you want him to gain weight , just like with us humans...more calories in, more weight on.  But are you sure you want to do that?  If you Vet said he's a good weight, I would think he would know.  In THAT area, Vets are usually correct. 

Here's a food charge that will help you with calories.  It doesn't list every brands, but lists quite a few:  http://www.catinfo.org/docs/FoodChartPublic9-22-12.pdf