Congrats! I love bath tub pics because the dogs look like they're in so much pain. Make sure you have the time commitment for Schutzhund if you're going to do it. The protection phase is the last thing you should be thinking about and just enjoy him as a deterrent. If he has what it takes for Schutzhund, then it is something that would be really fun to do depending on how committed you will be and how much time you have.
42 lbs for a 4 month old isn't out of the ordinary. It's large but not huge. I've realized 10 lbs a month for the first 6 or 7 is kind of the standard and then it starts to slow down a bit. Some dogs of course are less and some are more but in general its not the worst rule of thumb.