I think the thing is nobody can know 'how serious' it is.
Nobody can know whether he will catch up by himself, or catch up with support from school, or catch up with support from school and home, of if he won't catch up.
It is not possible to predict that.
But it's better to err on the side of caution - ie give him more support than he needs, rather than giving him less support.
And it is always going to be easier to catch up when you're 18 months behind, then when you're 2 or 3 years behind......
It's also easier to catch up before his self esteem is gone because he realises how behind he is.......
But I can tell you, it's a hard, hard, hard, road to catch up. I haven't managed it yet - DD is still very behind, despite an enormous amount of support from school and home and MN.
I wouldn't wish it on anyone.
And I certainly wouldn't recommend ignoring it, or trusting school, or thinking he's just young.....