Go research the term proliferative vitreoretinopathy (PVR). It's what's causing the reoccurring RD's.
With multiple surgeries the chances of recovering vision to the pre-RD state is pretty much nil. However, that's not to say that he won't have decent vision out of the eye. If the macula remains ERM (epiretinal membrane) free and has not detached at all and the optic nerve remains healthy then the majority of vision loss should be relegated to the periphery thus sparing some of his sight. Although, I don't want to give false hope, as the majority of cases the final visual acuity for extremely complex RD cases is poor, there is still a chance that things may not be so horrible.
At some point things should stabilize.
Remember to have your brother protect his good eye.
I too have had numerous surgeries for both my eyes due to RD, so I wish your brother the best of luck.
RD is a serious problem. There are risk factors: cataract surgery, positive family history, trauma, high myopia and of course bad luck. RD occurs in about 1 in 7500 people.
RD surgery is often complex, requires multiple procedures and the vision often does not return to normal. About one in 20 cannot be fixed.
you are at an increased risk because of your brother's problem. See an Eye MD annually. tell him/her about your family HX. Know the symptoms of a RD: sudden increase of floaters, flashes of light like lightening and loss of peripheral vision.
JCH MD