Worried... baby shaken during playtime

Grandpa likes to play "Earthquake" with my 6 month old son, where he holds him under his butt, in an upright sitting position. He slowly raises him up and jostles him around a bit and yells "Earthquake!" His feet dangle and swing and his head bobbles around. My son seems to love it because he laughs and smiles when they play. The first time they played the whole family was laughing and begging for more to see the big grin on my son's face. I'm a ftm and very over protective and by the book with him and even I didn't think much of it.

But the last time grandpa and baby played this game, my husband saw and said that he could cause shaken baby syndrome. After hearing him say it, I immediately felt like I let something terrible happen.

I've been reading everything I can on SBS and am so worried that some kind of internal damage was done, even tho my son has no obvious symptoms and is acting like his normal happy self. I want to tell my doc all of this but am also worried that grandpa could somehow get in trouble and he absolutely meant no harm.

Baby's head was unsupported, as he can sit upright by himself. His head did bobble from side to side and I remember telling grandpa "make sure you support his head!" right before my husband mentioned SBS. This is eating me up. Hubby says to let it go after telling grandpa no more playing earthquake. But I feel like the doctor needs to know just in case he sees something we don't.....

Has anyone been in a similar situation that can offer some advice? ? 😔