diet for a border terrier

[Ambypamby]

i have a little border terrier and usually he gets 2 cups of feed a day. he is very active as he is with me 24/7 and follows me when i ride often for miles. He has started to lose weight recently and although i put more food down he will eat a bit more than usual and thats it. he is wormed and flead. he is three years old. Basically i was wondering if anyone knew of any foods that encourage weight gain for fairly active dogs?

 

[KarynK]

With that much work have you considered changing to a raw or more natural diet?