Maybe he allows clients to do drugs, joins them or sells them drugs.....
Would you like to take chances on him? Is he's been doing drugs for a while his dick will be limp and he might be late, moody, unreliable or all of the above.
I'm sure there's a market for PNP just like there was a marked for BB before PrEP but some of us didn't take chances.
You can still play safe with him NOT doing drugs and asking him when was the last time he did drugs (and what kind) with a client so he can perform with you.
Obviously pot doesn't count as a hard drug.
Your choice!