AppleID recovery number changed

My sons iPhone 11 was stolen. I locked his phone immediately on my Find My Phone and I could track his phone using my phone. He then started tracking it on his old iPhone 6. We then blocked his number and phone and that's where the problem started. They changed his password and recovery number. Now i cant recover his icloud password. We can see the phone's location. It's in Maputo now. My problem is that i have no control over his phone no more it says no location found when i search for it on my phone. Luckily we can still trace it with my sons old phone. How do we recover his iCloud password after these thugs changed the recovery phone number