G.T.
If it's a blood blister than he should NOT puncture it. He could get a serious infection. It will heal and his body will absorb the blood on it's own. He can soak it in Epsom salts to ease any swelling and reduce the pain. Use the splint you got him to protect it from further injury.
Keep an eye on it. If it becomes red around it and swollen after a few days he may need to see a doctor.