My dog keeps licking one paw, it's red and he won't let me touch it.

Dogs over-lick for a few different reasons, so it's difficult to say what's causing it. Bobby may be over-licking the area because it's sore from an injury, has something like a thorn is stuck under the skin, or is infected.

I think it will be fine to try and treat this at home for now (even though we don't know the cause), but would recommend a visit to the vet if:

1) It doesn't get better in 2 weeks
2) It starts to bleed or there is a break in the skin

Here are my recommendations:

1) Wash/spray the area with just cool, clean water (no soap or other antiseptics) for at least 5 minutes at a time. Do this at least twice a day.
2) Ice the foot for 15 minutes once a day.
3) Tape a sock over the foot to prevent licking. You can also get some anti-lick sprays at the pet store to spray on the paw.

Once the skin looks normal and/or Bobby stops trying to lick, you can stop the cleaning/bandaging.

I hope that helps and Bobby gets better soon.