P.H.
I do not know the difference between PDD, but I wanted to speak to you as a parent of a preemie who has had many delays..you may feel that you wanted that RX and that speech is his only issue..but you need to bone up on everything they are giving you and start making all progress towards all of it..it is so hard having a child who is not on any of the charts, not speaking and having issues, but once you know what the issues are ..run with it and do what you need to.
I hope I am not making you defensive, I do not mean too, but you have 2 normal girls and a boy who needs all the extras to get him up to speed..my son did not walk until 19 mos, talk until near 3 and has had PT, OT & ST..he is 5 1/2 now and still gets OT & PT and he was only 6 weeks early!
Try not to look for the lesser RX, but see what fits him and his needs..speech will be wonderful as when he can fully talk to you and share his world..it makes so much difference.
Good Luck,