Generally, you can't claim clothing as an expense, unless the clothing is a uniform or protective gear. For a performer, they might be able to claim costumes, but not clothing that can be worn outside of work (tuxedos, business suits, etc...). If you are not sure if you can claim your son's clothing, then it would be a good idea to contact the Canada Revenue Agency (CRA) and confirm it with them.
If you are in Canada or the United States, you can call the CRA at 1-800-959-8281. If you are elsewhere, you can call 613-940-8495. This page has more ways to contact the CRA: Contact the Canada Revenue Agency
If you are able to claim the clothing, then you can enter it as an "Other Expense".
