If it were me--and this is just my opinion--if he likes the goat milk, I would continue giving him goat milk. There are many, many people who are allergic to cow milk (I'm one of them). The allergic reactions to cow milk/cow dairy products are many, and not many people/doctors are good at connecting health problems with cow dairy use.
I found out at the age of 40 years old that I have a casein allergy to cow dairy, an allergy I very likely had since I was born. It did a very, very bad number on my health. All of my health issues pointed at cow dairy/milk use--except none of the allopathic doctors I went to during my lifetime were able to see and make the connection. Why? Because they didn't know. I use goat and sheep products these days as well as alternative milk sources (like nuts) and have no issues.
That's just my opinion. I also believe goat milk has a lot more vitamins, minerals, etc., than cow milk does. I think if you do some research, you'll discover that that's true as well.