I taught a girl with this last year and coloured lenses really worked for her...they stopped letters jumping about on the page. I think it's just a case of trying different options out, but the specialist should be able to advise you if he's not getting on well with the overlays.
We didn't bother with overlays because our reason for getting the glasses was due to serious distress when using the white board. We had no idea there was any effect on her reading until we went to see about the coloured glasses. they have made an ENORMOUS difference to her. She got them at the start of the Easter holidays, by the end of the Easter holidays she had read 7 early reader chapter books, she was in reception at the time. I knew she could read harder stuff but she was exhausted after a page of a book but with the glasses she took off and stamina really increased. I don't think they are perfect, she still has problems but she rarely rubs her eyes now (she had been doing this all the time since starting school and had constant styes) and can read more comfortably, she doesn't mix the words up much now, if you ask her to spell out a word she is reading she can get all the letters in the right order which she can't without the glasses, she can see fullstops and she doesn't skip lines.
It is worth shopping around for them though, we went to a local vision therapy place and hers were a total cost for frames and lenses of about £150 I think, can't quite remember. there is no NHS prescription for them and although she is slightly longsighted it isn't enough for the optician to sign the NHS form so we had to pay the full amount.
What periwinkle said. But be careful - there's the Irlen system which I hear is a bit expensive/smoke and mirrors; what you need is colorimetry which is done by specialised opticians/optometrists. There is a good index on the British Dyslexia Assoc page.