Have you considered using a cup instead? Bottles are really not necessary for an 18mo. He'll be able to drink from a cup just fine by then, especially if he is practising already.
I would definitely freeze in small quantities like 3oz as he will only be taking it as a drink alongside solids by then, not a meal like he does now.
I never bottle fed but when I went away from my 15mo I pumped just over 15ozs in 24hrs so I would guess he took about 5oz per feed as he was on 3-4 BFs by then.
Have you seen our support thread for feeding babies 12mo+? You may get more relevant eyes if you ask on there or even post a link to this thread.
Extended Nursing Support Thread Part 5 Posted with last 2 pps...