this is just to ips so feel free to skip it if you want. as it may be long. 
ips not sure whether i should post or not really- dont want to scare you, but there might be a nugget of info in here (even if you only realise later)
anyway, i had trouble with chloe dd1 and constipation. she had terrible colic as well. i was given lactoluse and supposities too. eventually it seemed to get better. unfortunately at about the age of 2 she started to get kidney/water infections. they were almost chain like at some point- she would get rid of 1 to have another 1 a week later.
we were in and out of clinics, hospitals, different departments and consultants. she had kidney scans and dye injected as they were worried about scarring from so many infections. it was at its worst at 8 years old.
i used to despair. id ask her if she needed a wee and she would say no, only to pee on the floor 2 mins later. i was worried to death that she would get picked on at school. i would send clothes and i talked to the school about it. she used to change when she was wet, until one day someone asked her why she took a bag into the toilet. 
so we arranged with school for her to go in staff toilet out of view, but she kept coming home wet. im ashamed to admit it but i use to shout at her. then it transpired that because it was a different skirt, she wouldnt chsnge in case they noticed. so then i had to buy 5 skirts all the same.
the point of that it to show you how bad the effects were.
it took 6 years to finally be referred to a specialist doctor, who explained 6years of despair (i would cry over her, over the sheer volume of washing, over it being never ending) in an hour.
it was all related to her constipation as a baby. he explained that she had probably had a really hard poo and it had hurt her and she had sub/consciously hung on to poos after that. (im still convinced the supposities did it -pain) this in turn had made her bowel stretch and had the effect of her taking longer to go the the toilet as it took longer to feel full and have that i need a poo feeling, which just made it stay stretched.
as there is only so much room in a persons body, the stretched bowel was pushing into the bladder, causing all the infections and stopping her from feeling the need to wee.
he prescribed movicol. and told us it might take the same amount of time, ie, 6 years to unstretch it. the problem with movicol though, is it is a trial and error amount. too much makes them have accidents and too less doesnt help. she would also have very runny poos that wouldnt be a lot but that would soil her knickers and he explained that this was fresh poo that was behind constipated poo and was leaking down the sides.
unfortunately she is still affected. she has had 2 infections this year. im always finding dirty knickers in the bathroom bin and she has had to go back to pull ups in the last 2 monrhs, after years of being ok. ive been prescribed movicol but im too scared to give it her. im waiting for the school holidays.
i wouldnt use the supposities. im convinced i hurt her so bad that that is what caused it. i would try the movicol route as this wasnt an option given to us at the start. if you type in movicol in google there are literally 20/30 stories just like mine. lactulose is always the 1st option given to everyone.
i know you are struggling to give him that amount of water, and he doesnt always take the full ebm but i would check with the doc and if ok, divide it into ebm and a water pure apple juice solution. chloe would only drink it in apple juice. just be careful you dont put him off ebm though.
sorry if this is not helpful.