When you get him, make a drip line with your tank water into his little cup. When the cup is full, pour some out and keep dripping your tap water in until you think it's almost completely your tap water. This is called drip acclimation. If he begins to look stressed, you might tighten the knots or tie more knots to make the drips come out slower.
To make a drip like you just use an airline tube like a siphon, except tie a bunch of knots in it to slow down the flow to a drip.
Betas are pretty hardy though so I think if you do this when you bring him home, he should be fine