"Used to" needs to be followed by a verb because the "to" is actually part of the next verb's infinitive form. You could say: He used to be a boxer for five years. He used to box for five years. You will be understood perfectly if you use either of those constructions. However, it is technically incorrect to follow "used to ..." with a time period so it is better to say: He used to be a boxer. He used to box. He was a boxer for five years. He boxed for five years. This does not exclude frequency time phrases, only duration. Examples: He used to box every Sunday. He used to box for two hours but now he only boxes for one. You can't use it with "to be" + a noun (a boxer) because your state as a noun doesn't change that often. You are either a boxer, or you're not. You can use it with an adjective, though: He used to be tired after boxing but now he has more stamina. He used to be hungry in the morning but now he eats something before going to sleep. I hope this helps.

Michael and Parker gave excellent answers.

'He used to be a boxer for five years.' Used to + verb

Really, with a contained time period, "was" is a better choice. " "He was a boxer for five years.." "He used to be a boxer, years ago." In my opinion, of course. "He used to be a boxer for five years " ... this feels like an uncomfortable change of emphasis part way through, but I don't think it's forbidden. I just don't like it. Tricky ... we need La Liseuse :) https://learnenglish.britishcouncil.org/grammar/intermediate-to-upper-intermediate/used-to-infinitive-and-be-or-get-used-to-ing

'He used to BE a boxer for five years.'