Maybe he needs to try something less "sportsy" and more "artsy". Maybe he would enjoy getting involved in theater or taking some art classes or joining a children's choir or piano lessons? If his sisters excel in athletics then finding something completely different might be the push he needs to distinguish himself. If athletics is a priority in his family, then many cities and municipalities offer intramural leagues which might give him an opportunity to try out some sports his sisters might not have (I know our town offers flag football as well as t-ball/baseball and lacrosse in addition to soccer). At age five, swimming lessons could be considered an activity that could turn into his involvement in a swim team.
My son is not the most athletically inclined child either, but he is involved in martial arts. He isn't the most graceful student in his class, but we do encourage practice and celebrate improvement. Plus, five year olds are just awkward--they're going through growth spurts and they're not going to have the fine motor skills of an 8 or 9 year old. If I expected my 5-year-old to throw a punch like a 10 year old, I'd be disappointed. Helping him to realize that his sisters probably weren't as good at their activities when they started out as they are now might make him feel better (and if they were then he doesn't have to know all those gory details), and his seeing their improvement as they've practiced and grown might be the encouragement he needs.
Finally, and this is completely out of your control, it might be time for his mom to encourage his sisters to look at dropping an activity if scheduling is a real problem. You hate to do that, but if none of the children are old enough to drive themselves to practices and games and mom is the only current mode of transportation, then it's not fair to the youngest to say, "Oh, I know you're really interested in that, but sister X has this on Monday and sister Y has this on Wednesday". What you're really saying is "You're interests aren't as important as your sisters interests". Or maybe it's time for his sisters to start carpooling to practices with friends.