you're doing the right thing by seeking help and asking questions. The snake is obviously in it's best care of its life so far, but there is a bit of a learning curve. Hang in there... it's hard to get things right from the beginning, but once you get it, things stay pretty consistent.

Males don't normally get as large as females, and I've read where growth can be stunted a little, but I've also read where underfed snakes come around and grow to be the same size as any other adult. In my own personal opinion, I think a lot of it is genetics. Like humans, we're not all the same size, height, weight, and we grow at different rates. There are guidelines out there for growth, but we're all still different. Same with the snakes.

Feeding - if he's around 300 grams, an adult mouse may be on the small side. I would try to switch him to rats, if possible. Either rat pups or weaned rats. He'd probably do fine with weaned, if they are on the smaller size. I like to feed by the 10-15% rule... so I weigh my snake often and make sure her prey is 10-15% of her body weight. I fed every six days until she hit about 800g, now I feed about every 8-10 days. Some weeks, she doesn't take it, so then she goes until the next week. You have to find what works best for you.

Humidity - The towel trick is a great idea. You need a good size bath/beach towel for that. Some other things you can try... go to lowes and ask them to cut you a plexiglass top for your enclosure. If you have lights/heaters on top, make sure they cut openings for those, as the heat could melt the plastic. Maybe put some metal screen over those holes so the snake can't get out, but the heat won't melt anything. That will help keep moisture in. Or, you can wrap the top with plastic wrap or foil. Again, keep away from heat sources for obvious reasons. In addition, I would add more and larger water bowls. that will increase humidity. If you still can't get it in check, fold some paper towels and soak them in water and put them inside the hides. That is a simple way to increase humidity inside the hide where the snake spends most of it's time.