Red swollen thumb on 5 year old

Unless he's running a fever or there are red streaks beginning to move out into his hand and arm, I think waiting to keep your appointment will be fine.

It does sound a lot like an infection of some sort, and I'm sure the doctor will be able to diagnose and treat it without too much difficulty.

Put an ice pack on his finger if it's swollen. Did something bite him or did he jam his finger playing. Maybe the doctor will x-ray it to see if it's broken. Hope it's just a sprain. It will feel better after the doctor see's him. We'll pray for him. Good Luck. 

B"H

Could be an insect bite (like a spider), or he could have pulled a hang nail that got infected. Who knows. If you have animals, even a small hair could get under the skin like a splinter, but is difficult to see - even with 20/20 vision. You did the right thing to make a dr appt, especially if you don't know. Hope he heals quickly.  :-D