@alorinkaya
I think I might start by raising it with them as a question; "I'm sure he's mistaken, but Johnny said he sleeps in his clothes at your house?" and see what they say. Maybe that will lead to a dialogue in which you can say something along the lines of "Maybe if you reminded him to change to PJs & brush his teeth it would help him remember?".
Of the things you've listed I think brushing teeth is the most important. At 7 both my boys needed to be reminded and supervised in brushing teeth but if your parents won't, better a bad brush job than none at all.
But if you feel you really can't raise the issue, then what about giving your son a 'star chart' to take with him. Stars for brushing teeth, wearing pjs, combing his hair, and changing his underwear (at least). Either trust him to be honest or ask your parents to give him the stickers after he does these things.
The hair I leave to your discretion, you say it can't be done to 'self care' level, but why not? A 7 year old should be able to run a brush or comb through a #5 or 6 buzz.
Wearing the same clothes all weekend wouldn't really bother me as long as they weren't dirty or smelly. My now grown sons had 'favourite' articles of clothes (usually sport team or rock band things) that they wore days in a row. I figured it was a 'pick your battles' thing, but I always insisted on clean underwear.
The constant screens is a worry (for my two it would have been TV), but again pick your battles. Your career is 'notorious' for crazy hours and long shifts/weekends/holidays so your daycare options are limited. I can completely understand you not wanting to rock that boat!
As he gets older and more self-aware of personal hygiene I'm sure these things will improve. But for now pick your battles.