Magically appearing spots

Our JRT "Spike" is approaching his third birthday and he is suddenly developing lots of spots on his coat. From a coat that was mostly white he is starting to look like a little Dalmation (but with mostly brown spots).

Is it strange for a JRT to have such a change in his coat coloration at his age? Will it stop at some point (they are still developing it seems) or will our "boy" become more brown than white?

Thanks for any input.

Regards,