The cranial osteopathy won't have damaged / hurt that part of his brain. but you could take him back to the one you went to and see if there is further help? With DS1, when he grows the bones become tight. It interupts his sleep patterns, speech and moods.
Getting back to just the talking though, and teaching words. I was told once that a baby has to hear a word 500 times before it sinks in. We shout at our cat alot, and say "CAT!" thus explaining why one of DS2's first words was "cat"
DS2 is great at showing you his tongue if you ask "show us your tongue", or will give you 5 if you ask, however he shows no sign of saying any of the words related to it. Because he understands though I assume he'll get round to talking later.
Your DS is talking, and making an effort, maybe find one book he loves and really read it to him, concentrating on one word. See how long it is before he repeats it.
I understand your concerns, I thought my DS2 might have been uterring his big brothers name by now, but he's not, so we'll have to wait. He does chuckle though when I am cross with DS1 and use his full name, so he knows who he is!
How do you feel after reading all the posts? Has it given you any reassurance? BTW, what are your eldest 2? boys or girls?