I think I'm confused at how that's possible. First to 7 lairs wins. The rules require (see p. 4), that you build only ships next to lairs and lairs next to ships, and "You must always build in alternating fashion."
When you say "place more ships," I'm concerned you're treating them like roads in Catan. You can't just build unlimited ships and block opponents, like you could build unlimited roads and block opponents. (And in fact, in Catan, you could only build after you'd built two road lengths, so they in a way encouraged you to build lots of roads.) After you build a ship in Catan: Junior, you must build a lair next to that ship before putting another ship beyond it.
If your son raced out and literally tried to block you as fast as he could, he would build all 7 lairs and win the game. There's never a point in which it's even possible, I think, to block you unless he puts down all the lairs right away--in which case he's won.
When you say "place more ships," I'm concerned you're treating them like roads in Catan. You can't just build unlimited ships and block opponents, like you could build unlimited roads and block opponents. (And in fact, in Catan, you could only build after you'd built two road lengths, so they in a way encouraged you to build lots of roads.) After you build a ship in Catan: Junior, you must build a lair next to that ship before putting another ship beyond it.
If your son raced out and literally tried to block you as fast as he could, he would build all 7 lairs and win the game. There's never a point in which it's even possible, I think, to block you unless he puts down all the lairs right away--in which case he's won.