Hmmm...
...It could be that he's doing it out of nervousness or excitement. Has he done this before you've gotten home or soon after? If he's done it after you got home, it could be that he's just excited to see you and doesn't think about holding his bladder anymore. A lot of puppies go through a phase like that.
Also-- EVEN IF he does go in his crate, put him outside [I don't know if you're doing this or not] --for at least ten minutes. Being that the lab is still only 7 weeks old, he's probably still going through a phase. Most breeders don't sell their puppies until they are 8-12 weeks old because then the pups learn a lot about boundaries and even 'potty training' from their mothers. So his very young age could have a lot to do with it. He should eventually grow out of it- just remember to be certain that you do take him outside, even though he has gone in the crate. He's going to have to know his boundaries of where to go.
Generally, as they grow up, most dogs will not urinate in the place [or in A place] where they might sleep [who would want to do that, anyway? lol]. And watch him, encourage him to 'go potty' or whatever phrase you use to associate with it, and if he does it, try rewarding him. Positive reinforcement [such as a small treat or praise] will also eventually do the trick.
Good luck!