Puppy proof everything, and do not assume something is where he cannot reach it unless it's over your head!

Be prepared to keep him busy - a tired pup is a well-behaved pup. A shepherd mix will likely be smarter than your average Dane (no offense, I love them dearly, but in general they are NOT the rocket scientists of the dog world) so lots of activities, games, and toys, and never, ever, ever let him off leash unless you are in an enclosed space. A Great Dane can cover ground faster than you'd think, so best not to take chances!

It'll be interesting to see which side of the gene pool he favors, but good luck anyway!