He was probably weaned too early.  Better he nurse on the
other cats than on you.  It's very hard to explain the
resulting hickies with "My cat did that!"

Well I've got 4 month old kittens who are still nursing once a day. If he's
still that young, he is probably just 'comfort sucking'. Some babies take a
little longer to wean than others.I wouldn't worry about it, the mother will
eventually decide it's time for him to grow up and kick him off. I really
don't think you have to worry about psychological damage, if his brother and
mum get sick of it, they will get up and walk off. Is the one sucking the
younger/runt kitten?

It's totally normal behavior at that age - in fact, many domestic
cats never fully outgrow it. Please don't worry about it - if either
the mother cat or the brother get annoyed by it, they'll let him know!

They certainly won't suffer any psychological damage. Animals don't
have the same kind of body hangups that humans often have. To a cat,
getting annoyed by a suckling brother is no different from being
annoyed by another cat trying to clean its ears at the wrong time.
They'll just let him know, in a nice way (by leaving) or a not-so-nice
way (by hissing or bapping him on the head, etc, if he doesn't get
the hint).

You nailed it on the head - nobody seems to mind except the human
onlookers, who worry about these things! :)

Your babies sound really sweet.

Joyce

PS - I like your ID!