After careful examination...
We know his Up-B is an upward spiral punch, seen after missing wii fit trainer.
Likely his down-B is his counter move, to follow suit with both other counters and the similarity it has with blocking/dodging in punch out.
I think his standard B is a charged up punch seen a few times in the trailer. He backs down, can take some damage without flinching and punches forward. Attack seen during demonstration of power meter, as he is attacked by bowser. It seems that this attack is more of B move than a smash, considering the huge amount of non-flincing it allows.
His side B is likely hit attack that powered through megaman's charge shot. If you notice, he kept very fast forward momentum, which is unlike a jump attack. So it must either be his side b or his dash attack. The fact that the attack did have "a jump" and more non-flinching, makes it appear to be a side B instead of a dash attack.
I figured out exactly how his Power meter works while examining it's demonstration against bowser and megaman.
Begin:
1. mac is on the left with low damage and no powerbar, bowser mid with about 30% damage, megaman right attacking bowser
2. bowser is attacking little mac, who is using his charged punch.
3. Bowser does physical damage to little mac during this attack, but only to raise him to about 30%
4. as the camera zooms in close to the powerbar, there is about 1 or 2 frames you can pause on where you see that little mac's attack followed through and hit bowser.
5. the powerbar fills very quickly in the next second, mac's damage does not
6. as the camera zoom out from the powerbar, there is about 1 or 2 frames you can pause on where you see that bowser's damage is now about 230%
7. little mac unleashes his KO uppercut
so using facts 1 and 4 we can determine that little mac's particular attack is unrelated to charging the power bar. he has already finished it, and say "taking damage during that attack" is not true
using facts 1 2 3 and 5 prove that unlike Lucario getting stronger as he takes damage, the damage dealt to little mac does not charge his powerbar.
using facts 1, 5, and 6 makes it highly probably, that little mac increases his power bar by the damage he DEALS TO other players.
We know that filling this bar leads to a powerful uppercut, it is possible that the full powerbar could do other attacks, like the powerthrough attack used against megaman which I believe is his side B. Nothing can prove that this isnt the case ATM, but we have been shown the strong uppercut about 3 times in the footage, which would lead us to believe that it is the only result of the full powerbar attack.
Idk how you unleash it tho, maybe just by pressing B