Expressive language delay?

[Whyriskit]

DS2 is 2.2 and I'm concerned about his expressive language. He probably only has about 20 words, if that, but his comprehension is pretty good - he can follow quite a complicated request and seems to understand most of what I say to him. He can't say colours but can identify them, also things which are big or small etc. Gestures, points and remembers the signs I've taught him ("cake" is a particular favourite!). I'm also pretty sure he can physically pronounce the sounds as DS1 is doing a silly amount a lot of phonics at the moment and DS2 enjoys imitating him.
He shows empathy and his motor skills are ok, although he was a lateish walker (16 months). He gets a fair bit of 1-1 time as his older brother has just started school. He also goes to nursery 2 days a week when I work.
DH feels there's no need to ask the HV for a referral to SALT, but I'm concerned. OTOH, I don't know what they'll recommend that I'm not doing - I work with young people with SN and my mum used to be head of a pre-school language unit. Is it just a case of a late starter or maybe more to it? Does anyone have experience of something similar?
Previously posted somewhere else by accident - think this is the right place!