Yes, it is.  If your son cannot keep up with the other children in the class then it may put an undue burden on the teacher to have to constantly help your son keep up.  It can also be a very frustrating experience for your son since he will have significant problems with the coursework.  If this is the first time you son will be attending regular schooling then it could foster a negative opinion of school for years to come.  It would be better to enroll him at a level where he can succeed than to set him up for frustration and possible failure.

I understand your concern, however I do believe that it is fair not to pass him on. If your child did'nt pass the reading or the math test then the school should offer assistance for maybe summer school or even let him do a grade of 6th for a semester to see if he can proceed to the 7th grade that way your son will not be behind or just pasted on to a grade that he is not ready for.