There could be several reasons for what your child does.  These are a few examples. He could have problems with eye sight.  Have him checked, if you have not already done so.  He could become overwhelmed when the instruction is given to him too fast for his pace.  This often makes children nervous and they start making the same errors over and over again.  It could just be a growing phase.  
One recommendation would be to work with him at home and ask him to write one letter at a time and see if he reverses them.  Do the easy ones first like c, f, g, h, a, etc.  then go on with the hard ones such as d, p, b, etc.  
As a second step, ask him to copy short words - one at a time and see if he reverses them.  Try little ones such as me, cat, sat, pan, pat, can, and then give him words like Bob. dad, do, pat.  If you take your time with him and he does these activities well, then talk to the teacher and ask that more individualized instruction be given to your child or to slow down the instructions.  If the reversal of letters persist, ask to see the school psychologist or counselor, or reading specialist to asses your child.