It is a rather big needle that goes into you, but you're under anesthetics, so it'd not that big of a deal. Risks are minimal, and include unwanted damages to bone, muscle and nerves, plus any regular anesthetic-related effects. Realize that with any operation, these things are unavoidable and it is not unique to bone marrow donations.
Generally (quoted from Dal med school), there is about a 1% chance of adverse effects in the donor, and the 1% is almost entirely anesthetic related.
You will be sore for about two weeks. The amount taken, however, is very small, and it shouldn't really affect you all that much. All in all, it's worth saving his life--any life in general, really.