It looks pretty red but that could be just from him licking. I would give your vet a call tomorrow morning and discuss your concerns with them. The post operative care should be included in the price. I can't say I ever had a dog drop his butt down and rub on the gravel after a neuter. Has he been checked for parasites? I know the anal glands can refill quickly sometimes. Might be possible that they didn't get it all out too? I don't know but I always thing, better safe then sorry. I would give the vet a call. 
Your dog looks cute in his little e-collar, by the way.
Your dog looks cute in his little e-collar, by the way.