- And got the T-shirt
I'm not sure there's a way to know what would be best. The sooner you neuter him, the sooner he can go in with his harem, and I'm sure he'd be pleased about that. But waiting a little would give him a chance to get used to you before the trauma of surgery.
I think I'd wait on the surgery until I knew he was settling in well in a new home -- eating, coming out of his hideys, etc. The surgery will throw him out of his eating routine, and you wouldn't want to upset that apple cart if you could help it. But I wouldn't wait weeks.
And for his sake, I'd consider keeping him in a separate room from the ladies until you're ready to introduce them. If he can smell them, he may be pretty frantic to get to them.
Do make sure you use a good exotic vet who has had lots of experience with GP neuters. It's a somewhat different procedure than that for a dog or cat, and you want someone who knows what they're doing.