The rules vary by payer, but here's a short run-down (please check with your payers to be sure):
Medicare - you should contract the PA under his own rendering NPI (eg, the one that goes in 24J) - since he's part of a group, he will likely have the same NPI as the MD in 33a (billing NPI). You can bill the PA as the MD (using the MD's rendering NPI in 24J) under a few circumstances (called "incident-to") - it can't be for new patients, or acute conditions - basically, the only time you can bill the PA as the MD to Medicare, is when the MD has already initiated a treatment plan, and the patient is seeing the PA for a follow-up visit. The MD must also be physically present in the office suite at the TOS.
Medicaid: Varies by state...I can't help you without knowing where you're at.
Commercial: Many payers will allow you to bill under an MD's name, following the same type of rules as Incident-to; since the PA is an employee of the MD, it's allowed - but you'll need to check on an individual payer basis, to ensure that you're not violating any of your contracts.
The reason you'd bill under the MD's rendering NPI (versus the PA's), is that PA's are typically paid at a reduced rate (anywhere from 80%-90% of the MD fee schedule, depending on the payer); So even when you're allowed to bill under the MD's #'s (because you haven't contracted the PA yet, usually), you still have to do something to flag the claim, so that they know to apply the discount, in most cases. (Most want you to add a modifier, like the SA modifier, to your charges). Because of all of the hurdles involved in billing PA's under the MD's rendering info, it's usually better to bill under the PA's own #'s, when you can - that means that you'll need to make sure that the PA's contracted with the payers. If you need help finding the rules for specific payers, let me know what state, and I'll send you some links. Hope that helps!
