how can i translate

He is busy doing this, however he also enjoys doing this.

樂在其中 is an expression originated from 論語 (Sayings of Confucius) but 忙在其中 is a just word group likely 中補詞組. I would suggest a more literal translation as below: He is tied up with it and take pleasure in it.

Let me give it a try:
He is tied up with it and enjoy it at the same time.