chewing his paws/legs

Could it possibly be allergies?  I know some dogs can have an allergic reaction to grass or possiblity additives added to grass this time of year.  I know we thought our lab had this type of allergy.  (Turned out she became allergic to her food after 10 years..when we switched foods no more itchies []) 

Could easily be allergies -- and it could partly be contact oriented or just plain spring atopic allergies.  Dogs get MORE allergies as they age (where we humans tend to grow out of them, dogs grow IN to them). 
 
YES stop him from biting/chewing.  And you've got to get the skin healed up.  This type of behavior is almost immediately compulsive -- as the hair dries it itchies which makes them bite/chew more.
 
so go TO the dog calmly and tip it's head up and say 'no biting boy' and massage the area with your hands (that itself will serve to relieve the itchies a good deal). 
 
I would try Benedryl and see if it helps contain the itch.  1-2 mg/lb two to four times a day.  So the 25 mg over the counter capsule is the right dose for a 25 pound dog and give two to four of them a day.  Dogs need more than we do because their metabolisms are faster than ours.  This is one time a dog needs a higher dose than a human.  So a 75 pound lab needs THREE caps 2-4 times a day. 
 
You have to do this for a couple of weeks just to see if it is helping. Don't just give it once.  Continue it.
 
It will make them sleepy at first but they get used to it.  In the meanwhile you need to have a vet see the skin.
 
Is the dog scratching elsewhere??  Are there any tiny blisters or red pustules?  Do you see the edges of the dog's lips being red or sore??