Do I have to create a separate account to file my son's very first income tax return? He's now 18 but still my dependent as he has a disability (T2201 CRA approved).

Yes, you can prepare your son’s return in your account.

Or you can start a separate account for your son, so he can take over the account when he starts doing his own tax returns. 

But if you are using a multi-return package, then you will need to prepare your son's return in the same account where you used the activation code, or you will end up paying for his return separately.