He did ‘umrah but he did not shave his head, out of ignorance, and he put on his regular clothes

If a person does ‘umrah and does not shave
his head or cut his hair out of ignorance, then he puts on his regular
clothes, then when he learns of the ruling he has to take off his regular
clothes (and put on the ihram garments), and shave his head or cut his hair,
but he does not have to do anything (pay any penalty) for what he did of
prohibited actions such as wearing regular clothes or anything else,
according to the more correct opinion, because he was ignorant or unaware of
the ruling. But he has to repent to Allah, may He be exalted, for falling
short in learning what he needed to learn. The one who wants to do an act of
worship or engage in a transaction must learn the rulings on it; this comes
under the heading of knowledge that is obligatory for him to acquire. 

Shaykh Ibn Baaz (may Allah have mercy on
him) was asked: What is the ruling on one who forgets to shave his head or
cut his hair following ‘umrah and puts on tailored clothes, then he
remembers that he did not shave his head or cut his hair? 

He replied: 

The one who forgets to shave his head or
cut his hair following ‘umrah, so he does tawaaf and sa‘i, then he puts on
regular clothes before shaving his head or cutting his hair, has to take off
his regular clothes when he remembers that (and put on the ihram garments,
and shave his head or cut his hair, then put his regular clothes back on. If
he cuts his hair or shaved his head whilst wearing his regular clothes, out
of ignorance or because he forgot, then he does not have to do anything, and
that is acceptable; he does not have to repeat cutting the hair or shaving
the head. But when he realises, what he must do is change out of his regular
clothes so that he can shave his head or cut his hair whilst he is still in
ihram.

End quote from Fataawa ash-Shaykh Ibn
Baaz (17/436) 

And Allah knows best.