Super Speedy Spotting

Question
We adopted a three year old Jack Russell (Ralphie) about a month ago.  He's been nothing but the "good mood dude" we were hoping for.  So, his temperament and behavior are not much of a concern.  What I am mildly concerned about is the spotting that he's developed on his tummy.  I've done enough reading to understand that the spotting is normal and that these spots will continue to change throughout his life.  It just seems that other JRT owners see the spots appear over years, not within a few weeks.  When we first got him, his spots were very few and a faint blueish color.  Four weeks later, they're dark brown and spanning his entire tummy.

He's still cute no matter what, so it's not the aesthetics I'm concerned about.  Just hope there isn't an underlying health issue here.

Also, the vet mentioned problems with hips and knees for this breed later in life and suggested glucosamine supplements.  Have any suggestions for a good product?

Answer
Nikya,

Very sweet!! Congratulation on your new Jack!! It's great that his temperament and behavior are good!! Four weeks is a bit unusual for the spots to appear so quickly. Usually they are all "in" by the time they are your dogs age. He's a boy so he's obviously not pregnant which is one reason the spots change quickly. Any change in skin color can be a little worrisome so I would suggest taking him back to the vet just to be safe. It's probably nothing but it may be the beginning of allergies. If you are feeding a high quality 5 star food you don't have to worry about any supplements for a long time yet,  
Best of luck with your new good mood dude. :))