I've had a look at the formula tub and it's says that once weaning has been established, baby should be having between 500 and 600ml of milk a day.
Up until 3 weeks ago he was having 4 210ml bottles a day along with 3 meals, his schedule looked a bit like this:
7am bottle
9am breakfast - toast/weetabix/porridge and a bit of fruit
11am bottle
1pm lunch - sandwich/pasta with fruit and a cheese stick or biscuit
3pm bottle
5.30pm dinner - whatever dh and I have and a yoghurt
7.30pm bottle
This schedule and what he had varied with who had him before lockdown (3 days a week he was at nursery, with my mum or mil and dh and I are happy for them to feed him how they saw fit within reason)
He started leaving between 30 and 60ml in the bottle just after schools etc started closing roughly 4 weeks ago? so I reduced them to 150ml.
The past 2 days he has refused his afternoon bottle so his intake today and yesterday has been 450ml he has had more water though, we have a beaker of water in the livingroom that he picks up and drops through the day, which dh and I help him to drink from. He not quite grasped the tip up and suck yet 
I know his consumption is now more in line with the info on the tub, however, his actual food intake hasn't changed.
Is it normal for him to have dropped such a significant amount in a relatively short window of time?