Untapping/reusing

[Avodroc20]

Bare with me, and try to understand where I'm coming from.

{Otherworld Atlas} is in play with 6 counters on it. I tap it, me and my opponent each draw 6, then I play  {Hidden Strings} to untapped it and I also untapped a land. I then proceed to retap Otherworld Atlas to make us each draw 6, then play an additional Hidden Strings to untap Atlas and another mana. I tap Atlas one last time, we each draw 6, then I play what I want and discard till I have 7.
On my opponents turn, he attacks with everything he has, I don't block, and I take 8 damage. Then he proceeds to cast a card similar to Hidden Strings to untap his creature and tries to attack me a second time. (His untap card is an instant.) He argues the issue that he can legitimately attack again with that creature he untapped because of what I did with Otherworld Atlas. Please tell me he can only attack once because there is only one combat phase. It's different with artifacts because it's an ability that causes the artifact to tap.

[Millionlittlee]

There is only one combat phase which means only one damage step so this is not possible unless he has a card that gives him another combat step

RESOLVED

[Avodroc20]

Thank you.