I'm not a vet or a farrier or anything like that, but if I not mistaken, that would be similar to someone, a human, having fluid in your knee.
I had the problem often, my knee would swell and feel hot and uncomfortable, but I never really felt any pain. I had my knee drained by a doctor once and the other times it worked its way out through exercise.
I believe the same principle would apply to the horse. Since your farrier said regular exercise was in order, I would follow through with that. Work him on he lunge line and then feel for any tenderness. If he's not sore or tender after-words, I'm sure it'd be okay to ride him and work him under the saddle than on the ground if that's your preferred method.