haha, oh bless him, if he's survived this long I'm sure he'll keep going until it's time for the late summer moult (I assume robins moult the same as most birds, I'm sure the experts will be along to tell you).
Keep feeding him to build up his strength & hopefully he'll come back all fine & dandy. Don't worry if he's gone a while, we always think ours have deserted but they usually come back in the autumn, often with an attitude though....must be the new new designer look that does it.
I love robins, I've had some real characters over the years. One used to sit & scowl at me if I took my lunch into the garden...thing is, he hadn't quite mastered the art of camouflage - he alway sat in a bright yellow flowered shrub & stood out like a sore thumb!