Your rattie may have broken a toe bone, pulled a muscle, or he has an infection (this is very difficult to tell without seeing the rat). The best thing to do is keep your rat quiet and comfortable in a cage without things to climb. If you have a heating pad, set it on *low* under a corner of the cage so the rat can go there if it wants the extra warmth. If you see any discoloration or more swelling, or if your rattie starts squeaking when touched, then it would be a good idea to find a vet -- sometimes a break in a bone can require antibiotic therapy to bring the swelling down in the surrounding tissue. You can also try soaking the foot if the rat will let you. Get some betadine (iodine) solution, and using about a teaspoonful in a cup of warm water, a soak for 10 minutes twice a day should keep the outside of the foot clean and more comfortable while the inside gets a chance to heal.

If you don't have a rat-friendly vet, here are some recommended by rattie folks:
<http://www.rmca.org/Resources/vets.htm>
<http://www.ratfanclub.org/vetref.html>