I wonder if there is any potential disorder with a 6 years old boy who
as an expression of affection kisses his male classmates?

Can I read anything on that topic?

Thank you

My guess is there is no immediate cause for alarm. Calmly council the
child repeatedly that some or most of the boys will find his kissing
offensive even if they see him kissing the few who don't object. His
peers will likely soon convince him that you are giving good advice.  
Neil

I do not wish to frighten you.  This might not apply to your son.

I am gay.  Although I did not know what "gay" was when I was in
kindergarten, I knew I had "special" feelings for other boys.  I also
knew that it was not socially acceptable to kiss them.  You son could
possibly, potentially be gay.  This is not a disorder.  Do not attempt
to "deprogram" him as this will cause more harm than good and he will
resent you for trying.  As suggested by neilzero, do instruct him that
it is unaccptable to kiss other boys without their express consent.

Then again, this could just be a phase.

It depends too how "kissy" his family is - if they are a family who
normally communicate with lots of kissing and cuddling then this may
be a natural expression of affection for him - six is very little to
be making assumptions about sexuality. As Neil says, the other kids
will quickly teach him what is acceptable to them socially.