Getting a letter of recommendation when the referee gave no indication about the strength of the letter
Two things.

First, he is demonstrating strong willingness to write a letter for you. Although you can't know for sure without asking directly, this is a good indication that his letter will be strong. 

Second, ASK! In this situation, don't be afraid to ask him bluntly, as ff524 recommended. 

Finally, in future, you can avoid some of this uncertainty by phrasing your request this way: "Are you willing to write a strong letter of recommendation for X application?" In this way you are asking not just for a letter, but for a strong letter, and their response will be more indicative of the strength of their letter. 

And for this one, all indications point toward it being a risk worth taking. Best of luck on your application!