it doesnt look like an eye cap to me but the membrane that the eye cap attaches to. As for what to do about it I would suggest not messing with it and taking him to a vet. I dont know if some one else with more experience will be able to offer you an at home fix but as it is I wouldnt mess with it without knowing exactly what I was doing. Good luck getting a resolution and keep us posted.
I suppose it could be partially retained but the majority of his eye is very clear which is what makes me think it may not be the eye cap