I would speak to your HV about it. She has the power to refer you to a paediatrician and from there, they may refer you to an ENT specialist.
My first born DT always sounded like he had a permanent cold. I asked for a second opinion after doctor fobbed me off for the first 2 years. We saw a paediatrician who then referred us to an ENT specialist. It transpired DT1 had glue ear in both ears and problems with enlarged adenoids and tonsils. Just after my DTs turned 2 yrs of age, he had ENT surgery to remove adenoids, tonsils and fit grommets. He now speaks beautifully and as clear as a bell with no nasal sound in the background.
It's a simple process. Tell your HV you're concerned and that your kindergarten teacher is concerned. It may be nothing, but much better to get it checked now than to leave him with possibly problems that could be easily rectified BEFORE he gets to school.
Thanks. I'm not in the uk so I don't have a HV but will need to go straight to a specialist. Interestingly ds1 had a speech pronounciation issue that lead him to have a tiny amount of SLT which helped as did learning his alphabet but his speech is still fuzzy around the edges so perhaps it's a genetic thing. I think I may just get them both checked though I think ds1 has probably gone too far along to make much difference.