Ds friendship circle is changing a lot just now, he seems to be ok, but I can see one or two of his or the groups previous friends becoming a little isolated and their mums are worried.
In some cases it's because the dc regularly won't compromise if the rest of the group want to do something else, they refuse to engage with what the majority want to do and go home calling the other children mean or annoying (ime other children get fed up with this easily), then it gets to the point the child stops being involved and eventually the friendships diminish. They expect their friends to be loyal and battering down their door to get them to come out to play, but 11 years olds can be fickle and play with whoever is most fun at the time. Ds has been guilty of coming home if the group doesn't want to do what he wants, but thankfully not too often so far.
Another reason is when some of the group have more freedom/independence. The larger group want to spread their wings and don't want held back by the child who's parents won't let them go to the local shop/country park in a group. The child is embarrassed they aren't allowed so don't vocalise it to their friends and again start to lose the connection to the group or individuals in the group.
these are, like ds, usually only children, who need to learn the world doesnt always revolve around them or have parents who are more protective, or they are slightly behind in social maturity from the rest of the children. Hopefully they'll find new likeminded friends, or will reengage with the groups as they mature and are given more freedom.
I find ds, so far, stays connected with his closest friends by having sleepovers and inviting a couple of them to activities occasionally. If he was having trouble with his group I'd encourage him to invite over/out other children who are maybe of the same ilk.