A.P.
I have 2 sons also - a 3-1/2 year old and a 21 month old.
I'll bet your baby is teething. When my youngest went through this I would pick him up, give him Motrin and he and I would go back to sleep in the guest bed (so we didn't wake the whole family). He usually went right back to sleep since Motrin works so fast and cuddling with mom was an extra perk.
Also, my 3-1/2 year old has also been waking around 6 lately (used to sleep until 7:15). A week ago we put a digital clock in his room and told him that he had to stay in bed until the first # was 7. He's doing a great job with it. We told him that if he can't sleep anymore that he's allowed to get a toy and play, but has to stay in bed until "the 7". You may want to try it?
Good luck!