To flip or not to flip during ferber method check-ins (advice needed)

I have a 4.5 month boy that we are moving into his own crib and out of our bed.

We are trying out the ferber method.

He has learned to roll over from back to belly quite efficiently regardless of a sleep sack.

He doesn't love being on his stomach at times, but also prefers to he on his stomach to sleep at times. Little ones are fickle.

He is unable to roll front to back yet, so he cannot fix it without our help. We always start him on his back.

QUESTION: when we go in for our intervals and he is on his stomach bawling his eyes out - do we flip him to his back or let him stay on his stomach??