It would be reasonable to try again every few weeks or so. There should be calcium in the soy formula, so you shouldn't have to worry about him getting enough. You could also try giving him other milk products, such as cheese, yogurt, or ice cream. If he tolerates these, then there's no physical reason why he can't take milk, and the resistance is then more likely due to differences in taste.
If this is the case, you could try mixing a little milk into his formula, gradually going from mostly formula to mostly milk over a week or two, so that he gets used to the new taste. You should not use skim milk or low fat milk for babies this age.