You're taking good care of him
We had a similar problem with our little one a few weeks ago. We had been out at the park and visiting family, and when we got home, ChiChi was limping when we got out of the car!
We took her to the vet even though it was off and on because she was licking it like crazy and it was driving us nuts. The vet said it was a blister, not sure what caused it, but gave us medication to put on it and within a few days she seemed fine!
I hope your little Milo is feeling better soon.