No worries... You need to get him cleaned out first so stick with the Miralax for a week more at least a full dose of 2 tsp AM/1 tsp PM. If you need to make a smaller bottle of 3-4 ozs to make sure he takes it all, do it to get the Miralax into him. I had to do two small bottles on the chance that DD would only take a little... but had a second one ready if she wanted more formula.
Will he take a bottle with just warm water in it? I would try that a couple times a day as well. Best to watch out for high fibre foods until he ups his fluid intake as well - too much fibre without the water needed to use it can make constipation worse.. we are sticking with jarred green beans and pears here right now. Easy on the tummy and good for the pooper
If he is really reluctant to take formula, I would cut back on solids a fair bit to get his fluids back up... he really needs that right now to get everything moving... Still offer when you normally do, but limit him to half a jar for each meal. No cereal at all until that fluid intake comes up. No applesauce/carrots either - both are constipating. And of course, no rice or bananas.
I would try moving him to 3 bottles a day... with the lowered solids, he will get much hungrier and will be more likely to take a full bottle of 7-8 ozs. Another thing I have been doing since mine will generally take a full bottle without problems now is 7 ozs formula and an extra ounce of water (she will not take water in a bottle on it's own and is useless with her sippy). I also offer her a top up bottle before her AM nap since she would be going 3 hours with nothing after her breakfast... I use 2 ozs formula and 1 oz water... she still drinks it just fine and it gets a bit more water into her
With this method, she takes around 23-25 ozs of formula a day plus her 3 meals and sips of water throughout the day...
I have a recipe for the best prune baby food too - just wait to use it until you have a bit more fluid going into that little man
Simmer 10 dried pitted prunes, 14 dried apricots and 1 fresh pear (peeled, cored and cut into 4) in just enough water to almost cover the fruit. I bring it to a slow boil then reduce to low and cover for 25 mins. Let it cool in the fridge for an hour and then blend in until smooth. I freeze it in ice cube trays and give her 2-3 of them for breakfast at least every second day
She loves them!