There's no answer to this really, it all depends on if there's a cause (genetics etc) and then the therapies offered and when.
3 of mine have global delay, turns out the motor delays are hypermobility/low tone and CP(just for the youngest) related. All 3 have speech delay (6,3, 17m) and 2 it turns out have ASD also. For us its genetic, although we haven't narrowed down the reason ASD is in there.
Therapy helps so much as does just monitoring their progress via reports etc, seeing an up trend even if its slower.
When did Rory turn 3? My 3 year old is 4 in march, she is about 25% understandable in speech, with a couple attempts at 2 word utterances but just a year ago was non verbal, having had a full speech and sound regression at about 15ish months. Next to another 3 year old its glaringly obvious she is way behind but on her progress reports you can see a different picture. Letter aside, what have you seen progress wise as his mum?
Tests, usually some talks of genetic tests, sometimes MRI and/or blood tests.
You are right about speech delay affecting general delay, the frustration for a start affects their behavior, temper tantrums! Then there's the aspect where the social/emotional development cant progress as 'normal' because if you cant communicate vocally when your peers are then you're going to avoid or at least have a lot more difficulty.
My 6 year old with global delay is academically ahead now and aside from taking EVERYTHING literally he has normal vocabulary, his speech therapy is for pragmatic language delay. He had 1 word at 3. Motorwise, his EDS is severe so that's still years behind but known cause.
Keep on looking into the cause of Rory's delays and don't be afraid to ask questions
We're all here for you
Btw, my 6 year old son's name is Ruaridh, Scottish Rory, you have good taste