Hey guys, I am coming to you all for help

Little Stedman is too skinny IMO - and I want to fatten him up some. He is 4 lbs...but just seems so skinny to me for his body type. I can feel his little spine and hip bones and I DON'T like it one bit.
Stedman has honestly just never ever been a big eater (unlike his little OINK of a sister, Tatum

). Since the beginning of last week, I started giving him a pouch of soft/wet food every morning, and at first, he LOVED it - but now I guess he is "getting used to it" and won't even finish eating the entire pouch like he was in the beginning - and really those little pouches of food aren't even all that big to begin with
I do give him cottage cheese sometimes...but I don't want to give him TOO much because I don't want it to give him loose stools (although so far, it never has). I free feed Canidae (dry) and I have read other places that it is actually supposed to help put on weight - I don't know if this is true or not?? But it's not in Stedman's case, that's for sure!
Anyway...I am just wondering what you all recommend to put some weight on my little guy? I would give anything to get him up to 5 lbs - but he has weighed 4 lbs for the longest now...I highly doubt he will ever get up to 5

He is already a year and 1/2.
To be honest though, I really don't care what the SCALE says - I just DO NOT want to be able to feel his little spine/ribs and hip bones. Don't get me wrong - he is not like SUPER SUPER skinny or anything...but he COULD stand to gain some weight and have more meat on his bones
I am so used to chubby little Tate over here....I don't even know where her spine, ribs, and hip bones ARE lol. She is such the pig
Any help is greatly appreciated - thanks guys!