If you're able to use a 1 gallon jug and pre-mix the water, drain the container your fish is in (you can put him in a cup with some old water temporarily, I hope he's warm, that should help keep him comfortable
) and fill it up to the point you want to then gradually acclimatize him to the water. You can keep the rest of the mix later on (should he make progress and you want to top his water up) or another water change later.
He'll need daily 100% water changes while you treat him to replenish the salts. He may poo a little more than usual as magnesium sulphate/epsom salt acts as an internal cleanser, it may help ease his discomfort and remove the blockage.