Legitimate self-defense?

My youngest son, a CPUSA member, like his old dad, plans to march this weekend in one of the large May 1 parades in our city, with other hard-lefties. A local rightwing radio demagogue has threatened to show-up and "throw some punches" and break their antiwar and pro-amnesty (a hot local issue)signs over their heads. I have armed my son with a cannister of red-label (i.e., police strength) pepper spray, and advised him to carry one of his work instruments with him (he is a wood sculptor). Instead of fighting with threm,He prefers to work to form alliances with the working class Tea Partiers , a mostly decent bunch. But, is he and the others are attacked can he use these items to defend himself sand the others? The local police will try to keep both sides separate, but just in case they can't (we have a fairlyprogressive police chief), what are his rights to defend himself against th radio clown's threats (the goon has done such things in the past).