You are being scammed by accepting the money. Contact "him" immediately and let him know that you will not send any money to anyone and that he should not send you any money. Time is of the essence here.
You don't want to be knowingly receiving money that does not belong to you. Once you've done that, you are already going to be in trouble when the bank finds out that the money doesn't belong to you, which they definitely will because the person who it does belong to will want it back.
Do not contact this person any more under any circumstances other than to let them know that you don't want their money and won't do anything for them. You now know they are a scammer, an expert on scamming. You are not an expert on scamming and you will not be able to protect yourself from every possible scam they might try.
It is possible to scam a scammer, but it takes significant expertise. Many scams rely on the victim thinking they know what the scam is and believing that they are going to put one over on the scammer. This almost never works out well.
As you point out, you don't know how he'll react to you not buying the bitcoin. So you don't know what the scam is and what his angle is. So you won't be able to protect yourself.
Get out now. Get out completely. I hope it's not too late and you haven't already knowingly received money that you weren't entitled to.
