Here is how you will proceed:
1. After the Immigrant Visa is approved and granted, you may start planning to pay the physical Green Card (GC) fee of $220. It can be paid from time he is approved to before he departs.
2. If you choose to pay after he arrives the U.S. then the arrival of the GC begin to count from the date of payment.
3. Either way, it is usually better to pay departure. Upon arrival and when stamped in by the CBP officer, the green card process automatically activates.
4. While 90-10 days is usually posted to receive your physical card; and with no address change, you will get the green card within 4-6 weeks.