Thirteen is such a hard age. While it would be great if he stood up for himself, imagine how hard that is. Every child has a "place" in the pecking order and the reprocussions of "getting above your station" can be very hard and brutal. It is so painful to watch, I know, since we are going through something similar with our daughter. But expecting them to stand up for themselves is like asking them to throw the whole social order out of whack, a very tall order.
Have you tried talking to his coach? My daughter's soccer coach is wonderfully funny and supportive of those who aren't star players and his attitude towards them infuses the whole team. To him "heart" is as important as talent and he encourages everyone to shine in whatever way they can. He nips any fussing about mistakes in the bud, telling the team that anyone who doesn't make mistakes is not trying out anything new, the biggest mistake of all.
If that doesn't work, maybe he could find another sport he enjoys, something where his performance is less key to the sucess of the team as a whole. Running? Fencing? Swimming? Rowing? There are so many things he could try if his lacrosse team is making him feel clumsy and useless.