Yes, that turn bet was way too small. No, he didn’t have enough equity to call for the immediate 5 : 1 from the pot. However, he had implied odds, and it was a good call. Given he’s a fish, you don’t need to string him along. Get it in: 75% pot, or even pot by the turn. You know he has something he likes, and if he’s the type to take an (A,5-o) all the way, he’ll still call, and now he’ll be getting bad odds.
The other thing to remember about these fish is that a river raise is almost always the nuts, or very close to it. I’d absolutely hate it, but in cases like this, I can find the fold button.