OK - he needs to get himself on the SSAFA special needs newsletter list, he needs to register his son as a dependent with sn with CEAS, and he needs to speak to SCE who run all the schools in Germany. Where are they?
If his son has a dx, was statemented in the UK, then SCE would have evaluated the statement and discussed whether the SCE provision was appropriate, and put in support to make it so. or he would have got a UK posting.
Having a dependent with sn in the UK army CAN affect what posting you get (because of the child's needs, the army need to make sure that the family is provided for appropriately and do not end up where there is no provision, rendering them a welfare risk) but it will NOT affect promotion or career, unless the serving person is in a minority trade where he has reached a rank that they do not have any appropriate tours left.
If the son was not dx and statemented in the UK, then the SCE school would have carried out the statementing process - in this respect a parent has exactly the same right to request an assessment as they do in the UK from the LEA.
The problems happen because serving personnel erroneously believe that having a dependent with sn will damage their career, and they hide it. everything snowballs.
if the family have made the decision to HE, that is a totally different kettle of fish, and NOT the same thing as the army failing to make provision. The army never, ever, refuse to help arrange education for a dependent. if SCE said they could not provide ed because of the child's needs, then the family could have had a UK posting. if the family chose to proceed abroad and HE, fair play.
Happy to provide further advice to help sort this out. i assume they have already been in touch with SCE in Germany, and CEAS in UK? and have turned down a UK tour?
there's an additional needs conference for all services every two years - they've just missed one.