If we’re talking about when they were kids, he would have handed the tiny Sam over to Dean because he was too busy keeping them alive, and Dean would have still ended up as the one that watched over Sam until he woke up.

If we’re talking about when they’re older, probably some vase time, some grilling. But since it is Sam’s dad, and John changed less over the years compared to Dean, it’s likely Sam would have recognized him right off the bat. Cue some arguing and both snapping at each other until John realizes it really is Sam.

Unlike John, Dean wouldn’t be ignoring any phone calls from John about finding Sam again. He’d be there in a heartbeat, suspicious and hopeful at his core.