S gets this occasionally too - bottom lip out and cries seemingly in fright, it's horrible. I thought it was too young for SA as well but it's strange it's happening when he's not with you. Though saying that if he was grizzly when he woke up could he maybe be teething? Sam had little bouts of painful crying yesterday and was a bit unsettled and I think it was teething related as he also struggled to feed which is unusual. Calpol sorted him straight out.
The other thing is, are his GPs calm or quite in his face? My mum has made him cry a few times getting quite over excited with him and really in his face, particularly if he's just woke up or is ready for a nap. I feel bad for both her and S as she's just excited to see him, but I think is a bit overwhelming! S is also usually very smiley and easy going, so I think with him it's more individual reactions to a situation than stranger anxiety if that makes sense.
Would his Dad be happy to make changes / ask him mum to not be too all over him at first?