My 3yo is always tired too, I think it must be to do with their development. He sleeps for 12+ hours each night and naps during the day occasionally. I have spoken to friends with LO's the same age and they say the same.
Maybe your DS feels cold because he is feeling tired, I know I like to snuggle up when I am feeling a bit tired and run down. The mouth ulcer may just be related to a cold...? Maybe set aside some quiet time each day where he can snuggle up with a blanket and some books?