Aside from being a bit odd, that actually is quite adorable.
Are you completely sure the Silkie is a cockerel, not a pullet? Silkies can be very hard to sex! Has it crowed?
I have, on occasion, had my roosters attempt to mount my capons (castrated cockerels), presumably because they neither act nor look like males. Sometimes Silkies (and in particular, I imagine, a handicapped Silkie) can be quite difficult to determine gender based on looks and even behavior alone. Your rooster may simply think the Silkie is a hen.
I have also seen, after a fight occurred, dominant roosters mount the losers of the fight. But that tends to be more aggressive behavior and it would not make sense for a bird to be displaying both aggressive behavior as well as doing friendly things such as showing him food or laying near him.
Either way, if they are enjoying themselves, I say leave them. Odd friendships are the best kind.