Special milk for cats

I started giving Popsie milk that is treated with lactase so cats can digest it properly. I read that some times it can help with constipation and it makes giving his lactulose easier. I mix it right in with the milk. I've noticed that Popsie has started to gain some weight which is a good thing because he's such a skinny boy anyway. I'm wondering if it's the milk helping him put on weight because milk has fats in it. What do you guys think?

Cat milk is a food and not a drink, so yes, they will gain weight on it, and if you need to monitor his weight, his food should be reduced to take into account the extra he is getting from the milk - while he is skinny now, what if he needs lactulose long term, and he then starts getting weight issues?