We also use interceptor. Our vet put Roxey on it when she was around 4 mos. (the same time we had her spayed). And the price is pretty much the same as sisterdog said. We keep our dogs on it year round.
You should definately get him on it though! Better to be safe than sorry.

And I don't know about the giradia vacine. But we have given the dogs pills, and Huney was given a powder we put on her food for 3 days.