Well he either wants you to BULK up or he thinks the weight you gain (fat) is going to go into all his IDEAL places so that he can admire you like that. -_-
Since you're keeping fit, you've got the muscle and metabolism, if you want to increase in size you're just going to have to eat more and turn that into muscle in the places you want them but then you'd risk looking muscular (like too much). :P
Then on the other hand if you just eat more, I'm guessing most of the fat is going straight to your tummy and then wherever else fat tends to go for you.
If I put on weight (fat), it's not going to go to my assets first. I have a little fat on my tummy but I'm quite thin everywhere else. If I lose a few kg I will have a flat tummy for sure but then I will truly be thin because my frame is quite small. Then only will I agree with people calling me skinny.
I have people telling me to gain weight, but I find it annoying because when they say "gain weight" they're not really telling me to be HEALTHY. Just because I'm small doesn't mean my fat goes in all the 'right' places. If the bulk of it goes to my waistline and tummy, which it does, I'm sure as hell not listening to them.
The other option for me is to just gain muscle weight but I do not want to have muscular thighs and all that kind of stuff, so really, I'm just going to stay the way I am. It's ridiculous.