being 19 months, he is likely to have been tested for deafness with the neonatal screening programme, but that doesn't mean that he hasn't developed a hearing loss since then. does he have a lot of colds/stuffiness?
deaf children's spoken language tends to develop like hearing children up to the point of babbling, but may stall at this point without intervention, so his chattering may indicate a hearing loss. also the noisiness you mention is often found in deaf children who are not aware of the noises they are making, such as noisy breathing, etc.
you could suggest asking gp for a referral to an audiologist to rule out any hearing problems. if mum is anxious perhaps you could emphasise that temporary hearing loss (such as 'glue ear') is quite common in young children and that treatments are available.
Some of things you mention sound potentially concerning - although it might just be the way you have described them. Is there also an absence of expected social gestures - waving, pointing for attention? Or are you confident that it is just a hearing issue?