He is 19m has 200ml cup on waking, normally asks for more so gets another 100ml then has porridge made with another 200mlish.
Throughout the day he eats really well but not keen on veg, his 5 a day come from fruit unless he has spag Bol which I hide toms, onion and mushrooms.
He drinks a bit of water a day but not enough I don't think and always asks for milk. I try and persuade him with water but when he goes into complete melt down for milk having not had much fluid, he'll get another 200ml. He's now asking for more, dinner will be in 20 mons and he could well take another 200ml before bed at 7ish.
He plots the 9th centile for weight but 75th for height and sleeps really well, so is a litre of milk too much?
Yes they can have too much, it can rot your teeth, doesn't flush your kidneys through, and stop kids eating well as they fill up on milk, big evening feeds can stop kids learning to be dry at night. You could try watering it down, or putting a limited daily amount in a big bottle in the fridge and explaining that's the whole days supply, OR some form of bribery re a shiny new water bottle? Sounds like he's eating and growing well though, so don't stress too much, but if I were you I'd try to get more water and less milk into him.
This was a real phase with my nephew from about that age to the point where his mum used to carry a 4 pint bottle of blue top around with her. It did of course stop him eating much solid food but he was much worse than your ds who is eating a reasonable diet. He snapped out of it and started eating more solids but it took a long time and maybe it could have been curtailed earlier but that's just my opinion.
You could either dilute it or you could buy a really small beaker/sippy cup or whatever he drinks out of and limit it that way. Or you could fun and tell him there's no more after a certain point. This still works with dd as she can't see in the fridge
I agree re the fancy water bottle too - always worth a try