Helmets will be no problem. 54cm is merely a small (Size S) helmet for most manufacturers. Buy the one that fits best. Don't get a second hand helmet off someone. Please. Make sure it fits. If it doesn't it will merely transfer the force of the impact to his head, which is what the helmet is trying to avoid. All Helmets have to meet the same minimum safety standards to be sold, so don't let salesmen guilt you into a more expensive helmet.
Check out the bike shops for Women's gear, at that size and weight he may find getting a jacket difficult. Draggin make smaller size motorcycle jeans too. They have a kevlar liner in the usual impact points and having tested them I can say they work. You can get impact foam for them too.
Bookmarks