tmr my ds had the same issue. His sister used to speak for him & he was using signs for everything, only a few words. I tried to get help from the health visitor when he was 2 and I'm still waiting for them to get back to me; he is 8 now.
Because of that delay we had to wait until he started nursery for him to have some support and that takes time also. So as a result he's reading was also delayed & he's still trying to catch up with other kids in her class.
I'm saying this to you because sometimes we think they'll be fine and catch up; but others it doesn't happen as fast & the others keep going as they don't have the same issue. There's no harm in your son attending those sessions. If he doesn't need them, they'll stop it but kids need to start speaking at about 2 or they get too comfortable in their silence, if you know what I mean.
Hope you won't be offended by my comments. You know your child better & I suffered a lot with mine so I know how much it hurts when people think there's something wrong with him & they judge without knowing.
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