i think the key is to let go and relaaaaaaaaaaaaax !!
if he is still having a bottle at 18 months, the sky won;t fall in ! i started to introduce a sippy cup of water with meals at the start of weaning, but took DD until she was 9 months to actually drink properly out of it, to quench her thirst, rather than play with it
also, some children just don;t eat a lot , regardless of age, and regardless of BLW or puree weaning
DD is 21 months and has eaten small amounts from the get go, and when she is poorly or teething, will not eat a meal, wants milk, and that is all........
so as long as she is alert, weeing and pooing, developmentally she is meeting her milestones, i don;t stress too much about how much / little she is eating
children do not starve themselves
some prefer 3 big meals a day, some prefer 6 small meals, you will get to know your baby and their requirements