Question: 5month old axoloti

What do you feed them .and how often

Depends what he was fed on previously. You can maybe feed small earth worms. Frozen And live blood worms, white worms, depends on how dep your tank he may still notice daphania, and brine sharpies, axolotl pellets and reptomin.
How often, again depends on if you just got him how often he was being fed before. I'd says maybe 2-3 items a week, and as much as he can eat in about 5 minutes or so.

At five months, you can be feeding them earthworms, which are the best staple diet for axolotls. You can try them with live, whole earthworms, if the worms are not too big, or you can cut the worms into bite-size pieces (about as long as the axie's head is wide). If they don't like the taste the worm excretes, you can blanch the worms (dash them in hot water, then quickly in cold so you don't actually cook them).
You can buy earthworms from bait stores, dig them from your own garden as long as you know no pesticides or other contaminants have been used there, or start your own worm farm (there's a great sticky on this site about that).
At that age, you can feed them daily, pretty much whatever they want to eat (if they throw up food, you can give them a bit less). Basically, you're aiming for the tummy to be about the same width as the axie's head at the widest point, but as juvenile axies tend to use their food for growing rather than putting on weight, they can be quite greedy.
Enjoy your li'l lotl.