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It could be due to various reasons; maybe he's thirsty, had a bad dream, or is teething. Try offering him some water but let him stay in bed. Also, don't talk to him too much at that moment, and after he drinks, just leave him alone and crawl back into bed. This way, he'll learn that waking up in the dark doesn't mean he can get out of bed.
It's quite possible that he's just tired from all the stimulation at your in-laws' place, which is why he's sleeping longer.
It's a phase that will pass.

Oh because now it's going to be hell.

Be glad that he has slept so well for so long. Maybe he's cold, maybe he's bothered by the hour, hungry, thirsty.

Bring him over to you guys.

My 9-month-old son wakes up 3 times a night. And sometimes I hold him for 2 to 3 hours until he falls asleep again. You need patience for such a little one. It's pretty tough. Otherwise, check if something's wrong. Cold, teething, or something. Could also be colic. Good luck, girl. Don't lose hope. Before you know it, the little one will sleep longer. Or take naps too, so you get some rest too.

Just let her cry it out. My youngest turned 2 and hasn’t slept through the night for even 10 nights. Sleeping through the night is really a luxury. She’ll probably still wake up 5 times a night, even though she sleeps between us.

My two older kids were 5 and 4 before they started sleeping through the night in their own beds.

Make it as easy as possible. It could also just be that he’s done sleeping by 5 am. That’s been the norm here for years.

Try tilting the crib a bit. Have the doctor check his ears. I totally get that it's exhausting with 5 hours every day, and yeah, sleeping through the night is a luxury, but who wouldn't want that... I'd first check everything out before you keep putting him in your bed, so really feeling well and checking his mouth is important at this teething age.
Check if the daytime pattern is too busy for him nowadays?
And as mentioned above, change the water bottle in bed, don't talk too much to him! Does he have a cuddly toy in bed?