Were I faced with this decision there would be other factors I'd want to consider, but I've seen it done both ways.  In general, though, in my experience (having raised four kids through this age) there are few if any consequences of holding your child back, whether it be holding him back in preschool this year or holding him back in kindergarten after this year.  The slight difference in age between him and his classmates isn't going to be significant...my younger daughter is the second oldest in has class (now fifth grade) and likes the fact that she's nearly the tallest.
 
Leonard Sax has written a couple of good books on the differences between boys and girls and how they develop differently; these might give you some comfort in a decision to hold your son back.  But, again, so much depends on individual differences.
 
Education.com has a good section on gender differences and kindergarten readiness.  It might be good for you to spend some quiet time doing some reading in these areas.  Links are below.
 
Hope this helps.

Very often when a child shows signs of speech issues they will eventually also show signs of learning difficulties.  This is because speech and language relate to reading and writing.  If a child isn't hearing or pronouncing their sounds correctly then they will have extreme difficulties decoding letters and sounds.  In my opinion another year of preschool is not going to change anything.  Instead I would focus on intervention immediately.  As he goes through Kindergarten - make sure he continues in speech therapy and also make sure he gets remediation in learning his decoding skills in reading.  If you get him the help he needs then he will do fine continuing in school. Another year of the same thing that didn't work last year isn't the answer.

As a teacher and a parent, I can totally understand your dilemma. At my school we screen kids just before school begins, and we do not tell parents a child is "not ready" for school for many reasons. Kids sometimes don't perform their best when they are nervous or anxious about performing for a stranger.  You know your child best and how he might function at school.  What does the speech therapist think?  Does your child have behavioral issues you are concerned about? Turning 5 in May is definitely not too young; the youngest Kindergarten student in my class turned 5 in November after starting kindergarten.
Here is an article that weighs in on the various factors of kindergarten readiness. Good luck to you and remember: you know your child better than someone in a 15 minute interview!

My son will turn 5 mid July and we have decided to register him for kindergarten for the fall. He did has a pre-screening and they thought he did well. The one thing they told us was that they were suprised at how well he did socially for being at young 5 for the starting year. The teacher said that if they are not socially ready that would make their school year that much harder on the child. It seems like that is what son may have experienced. Try enrolling him in some activities that he would have to interact with other children without one of the parents being around, like sunday school or something like that.