For me, anything that you pay him IS part of his "bill", and might be considered a deposit/ down payment. Considered still part of the agreed total, only delivered partially in advance.
To me, three weeks in advance seems quite far out, especially for someone who is new to that field without much of a reputation, reviews to check, a history as a provider, or someone who you haven't actually worked with before.
Agreed, a lot can happen in that time.