Even if it only takes him a month, he's still behind where the other summons started. I'm sure they aren't resting on their laurels. Don't think he'll be catching up any time soon.
Did the walking math: just to lvl 20 > 25, he needs to walk from Boston to Chicago (or Paris to Warsaw, Poland).
The heroes he summoned alongside started with stats equivalent to what he'll have at lvl 29.
Better start walking.