Your brother's $650 payment is in the correct range to be a SSI payment, rather than a SSDI payment.

If he's 42 now and the accident about 15 years ago, he would have been approximately 27 then. The number of quarters he had in (most likely under 40 unless he was working continuously from 17 on) and what his earnings were would contribute to his SSDI rate. As low as his rate is (SSI range) it looks like he didn't work much before his accident if this is his SSDI rate.