My twelve year old son has a rash on his feet and palms of his hands. He had a mouth ulcer last week but no other mouth sores. He also has mild cold symptoms. Is HFM in winter possible?

Hand foot and mouth . Yes HFM is mostly a summer virus but the virus does not follow the rules. Your description fits very well. The blisters last 1-2 weeks in the hands and feet. The oral sores are earlier and go away after a week. The rash is contagious if the vesicles are opened and drained. There is no treatment. Pain meds do help if needed. Drink lots of fluids. Good diagnosis !! .
See pediatrician. Yes, go to your pediatrician for a definitive diagnosis.