paulflexesOnly one space after the period is the current norm; but you should have used a semicolon before but because it's a separate statement that requires less of a pause than a period when speaking it.
Eh? That's not why you use semicolons. Actually, he can use the comma there if he really prefers but it's not at all required.
A semicolon's purpose is to link two independent clauses without using any words to join the two. It has nothing to do with pauses when reading. (The other use is for when you have joining words like however, therefore, etc...)
I walked my dog today; he sometimes chases after rabbits.
I walked my dog today even though he sometimes chases after rabbits.
I walked my dog today;however, this time he didn't get to chase any rabbits.
Sorry, my grammar nazi is showing
Oh and for the OP: like someone else said, two is a left-over from the typewriter days. Most word processors will now actually properly pad the space after a period so the second space is unnecessary. It's still accepted, but one is becoming the convention again.