Several years ago I trained a 4yr old that had an incredible natural affinity and aptitude for boxing. His appetite was voracious, his attention span that of a 4yr old, plus he was a bit of a know it all. The trick was to get him to believe that something new was his idea and to make sure it worked and he'd have it for good.
Later I trained a 6yr old who was only there because his granfather made him be there. He had no interest and thus we had no basis for communication so we ended up playing games until quitting time. Perhaps a few years later perhaps not. The sport isn't for everyone.
I think Thomas is correct in highlighting the work and so on involved in boxing. I found it worked well to make it a game that you had to practice very very hard in order to play well. The most progress I made was after "training" when I'd get on my knees and let the 4yr old take his shots. If he was just winging and playing I'd not let him hit me. But if he used his jab and moved aroundand threw "combinations" I'd let him clip me a few.
Its worth keeping in mind that at 5 he hasn't a fight scheduled. Its just a game and all in fun; if that's his interest let it be that. If he wants morego with it but never push it beyond his willingness to play along. Too young to learn about drudgery.