Dogs stop growing at different ages, depending
on breed. Larger
dogs stop growing later, smaller
dogs earlier. For a Pointer, it will probably be around 1 year, or maybe a little more.
Don't you worry about him being a runt! Unless you intend to show him, it won't matter. As long as he is well-nourished through the growing period (and of course well cared-for after) he should grow to be perfectly fit and well. If he's a little smaller than his breed, does that matter?