No research necessary----everything is in this thread already.
1. Get your ICAO license valid again---it is useless if it is expired.
2. Get an ICAO IR.
3. Pass the FAA ATP written and flight test, (which will include the IR).
You miss the whole point---his Serbian CPL has EXPIRED....as he stated in post 1
got ICAO CPL(H) with type rating for Schweizer300, which expired but I can renew it
Therefore he needs to re-validate that first. He NEVER did have to do an FAA commercial.
He can then obtain the foreign IR...then he meets the requirements of FAR 61.1523(d)(3) and can then do the ATP without having to do an FAA commercial.
As Birddog stated, he will need TSA approval.