What to do with serverly frostbitten toes on 9 week old chick?

The main thing is to not let him out until all chance of any more frostbite are over. Repeatedly getting frostbitten will result in loss of toes and/or the feet.

Warm him up to normal and you can warm his feet/toes by using warm, not hot water.
Do not rub the area frostbitten or pop any blisters.
Clean in mild diluted betadine solution. Should look like weak tea color.
Wrap the legs and toes in sterile bandaging that's DRY. Use antibiotic ointment to keep areas from sticking to bandages and try as much as you can to not move the thawed areas as it's very painful and can cause more damage.

Since he's been frostbitten he will be more prone to repeat and will need to be brought in if it's going to get into freezing temps after he heals from this episode.

You should give him some extra vitamins<Poly Vi Sol w/OUT iron> and extra protein to help build back tissue and heal. Yogurt in oatmeal or starter mash, and ACV and Garlic in his water will help as well.

If you start seeing infection or even as a prophylactic measure you may want to put him on antibiotics in his water as well.