It was totally inappropriate of him to share his opinions like this. Very unprofessional, and has undermined your mum's trust in him. Not all help is useful, and some can be downright damaging.
For context, we had a massive battle to get DC assessed, the schools, LA and the local NHS Trust put up lots of unnecessary barriers because my DC does not present in the way expected of a young boy with his profile (masking, AuDHD & Sensory Processing Disorder). Got there eventually, so the time, effort and cost is slowly paying off in better provision and adjustments at school (mainstream).
What issues are school and your mother seeing that cause concern? What support was she/DB hoping to get from the counselling charity?
I would not subject my DC to such a counsellor for a further session, tbh he sounds out of his depth in dealing with suspected neuro developmental issues, and highly unprofessional to be so dismissive before getting to know DB and his stuation. I would write a formal complaint to both the charity (his manager, as well as the Trustees) and the referring body (was this school? LA?) outlining what was said and questioning if this is considered suitable behaviour, and that it has now setback your mum's attempts to get help and support for his difficulties.
If you can afford to have a clinical psychologist assess him, that's what I'd get done first. Find a well qualified professional who has experience with diagnosing neuro develoomental conditions and in writing medico-legal reports. Once you have this you can decide on next steps. If this is not affordable, you can wait to get the autism assessment on the NHS, and ask for a second opinion with regards to ADHD traits. Just a community nurse observation is not good enough. At the very least the Conners ADHD Scale questionnaire should have been given to mum, DB and the school (teachers who work closely with him) to fill out. If you can get an appointment with a,specialised ADHD nurse, that might be more useful. However, there is only one in our whole Trust and we could only access her post diagnosis, but it might be different where you are. Find out what the procedure for ADHD assessment referal is in your NHS Trust, and get your DB on the pathway if there is evidence of ADHD/ADD traits.
I would be looking up national charities that support children with SEND/ALN & their families. They can give advice on the getting support and what LA legal duties are etc. Some have people specialising in advocacy that will attend LA/school meetings with the parent.
Which UK nation are you in? The rules and pathways are different in all of them, so you need to know this before you start.