Oh I know Molly, it's such a minefield!
Reading your OP again, you mention a friend who is a speech therapist - she would be a good option for the 'somebody who knows your child' because, even though she'd be writing from a personal perspective, putting her profession gives her some clout - as in, she knows what she's talking about as she's a medical professional. The fact that she knows him/you on a personal level and can see that there are additional needs that he has, makes her a good 'somebody' to know, if you get my drift.
I can't stress enough, do put your GP down - they will have contact (mine did at least) with your Health Visitor plus, of course, list your Health Visitor! A letter from her would also help - don't wait to be asked for additional things like evidence, letters, etc. - send them in with the form straight away. There's never too much evidence for this kind of situation.
My daughter failed her mobility on her last DLA 3 years ago, but it was overturned the minute a SW from the Disabled Children's Team wrote a brief letter. You'd be surprised how small things add up when it comes to DLA!
If you do need to get help filling in the form, see if there's a Carer's Centre near you or, failing that, the Citizens Advice - although with all the cuts recently they are overrun but will help.
Do, though, read through the Benefits & Welfare guide - it's a fantastic aid. :)