Praise be to Allah.
This question was put to Shaykh Muhammad ibn ‘Uthaymeen (may Allaah have mercy on him) and he said:
If he did that out of ignorance, then he should take off his regular clothing (and put on ihraam) and shave his head completely or cut his hair, and what he did will be forgiven because he was ignorant. Shaving or cutting the hair does not have to be done in Makkah; it may be done in Makkah or elsewhere.
But if what he did was based on a fatwa from one of the scholars, then he does not have to do anything, because Allaah says:
“So ask of those who know the Scripture, if you know not”
and some of the scholars think that cutting hair from part of the head is like cutting hair from all of the head.