Great Dane Care

Question
My Great Dane puppy is 11 wks old and yesterday started showing signs of his back right leg being sore.  He does not shows signs of pain when we touch or straighten his leg, but is slower to get up and keeps pressure off of the leg when standing.  That night he started getting rowdy with my older MinPin and am hoping it is nothing serious and just a minor thing that will heal in a couple days.  
All of the things that I have researched about Danes have made me soooo paranoid and I am getting anal on trying to make sure that I am doing everything right by this puppy and his health.
I got him 3 wks ago and started walking him on very short ten to fifteen minutes walks at the park in my backyard.  I do not put the leash on him so that he has free rein to play, run, and skip around. I heard that a short walk everyday will help his bones to become stronger, but now I am worried that I have hurt him.
I am feeding him Purina One Lamb and Rice because I heard this was the best supermarket brand and this weekend I will travel to the PetSmart to get a better brand like Eagle Pack.  I heard that this was a good brand for Danes.
Please help with some good advice on making sure his bones grow steadily and to prevent any serious bone damage.

Answer
Get the vet to check him over.

Never used eagle but Pirina is used by many dane people.  If you live in the Midlands you could use what we use, TPMS from Redditch or get Gilpa Dr Johns Silver or Gold.

Keep away from the very expensive, and overpriced, foods.

Ken