If he's late, sounds like he's hiding out until the worst is over. Sounds like he ought to get up earlier but is avoiding doing so.
Seriously though, would he help properly, are you sure he would not just get in the way? think about it?
Does your 2yo need to go out? or is that only because you take them on the school run too?
If the 5yo is the priority, then get them ready first, straight after breakfast.
Could DH then dress the 2yo?
Or seeing as uniforms are easy enough, could he do the 5yo while you get the 2yo sorted?
They are his DC too, you could do with a hand, he could do with getting up earlier, you could help each other. Done ask him will you, ask him which one he can manage while you do the other one. Tell him you'll pay him in kind if it helps 
Establish and communicate a simple and easy structure to your mornings, breakfast, change, Mum gets ready, they watch telly. Out you go. It's what I had to switch too, cos it works better that way, for me.
Deep breaths, it's trial and error with school mornings!