Hello, I'm Dr. Deb and will do my best to help you today.
I'm sorry that your pup hurt himself although this is actually a fairly commonly seen problem in dogs this age:(
It's quite possible that he's experienced a sprain or strain or pulled muscle or he might even be bruised since you can't localize an area of pain
The following is what I'd suggest you do:
1. Cool compresses can be applied to the shoulder area but be careful to only apply them for 5-10 minutes at a time. They should be wrapped in a towel or something similar since it's possible to burn the skin if ice is directly applied to a dog's body.
2. If he's not currently taking any medication and not vomiting (I assume), he could given Aspirin at a dose of 5-10 mgs per pound of body weight twice a day; this drug should be given with food to avoid stomach upset.
I'm sometimes hesitant to advise its use since it might interfer ewith what your vet would want to prescribe (if he has to be seen) but one or two doses will not be harmful. The anti-inflammatory as well as anti-pain properties could help in this situation even though he's not actively showing signs of pain.
3. Leash walk him only when she's outside for a minimum of 7-10 days. Don't let him run around or he may cause more injury. It may be hard to keep him from jumping off and on the furniture inside (if he's allowed to do this) but do the best you can in preventing it.
Puppies at this age tend to heal pretty quickly but if he's worse and/or there's no improvement in a day or so, then a vet visit may be prudent
I hope this helps. Deb