You are right, "workaholic" is not the best word because of it's negative connotations of compulsiveness and lack of time to do anything besides work.

I would use something like "a dedicated hard worker" or "a committed hard worker" to describe that person.

committed: willing to work very hard at something SYN dedicated

(Longman)

"Workaholic" is indeed not the best word because of it's negative connotations of compulsiveness and lack of time to do anything besides work.

I would use words like "dedicated", "committed", or "hard worker" to describe that person:

committed: willing to work very hard at something SYN dedicated

(Longman)

Consider:

He is extremely committed.