I would say it depends on whether you want to do multiple TSMM or multiple RSR rides because to get multiple RSR FP, you have to be among the very first guests to get a RSR FP in the morning. FP times start about 45 minutes after the park opens (so on Thursday, it'll be around 9:45-10:45). So you would have to get the FP and then go back and get another when the first FP window opens up. RSR FPs do seem to be lasting longer since they no longer allow guests to line up pre-park opening, but I wouldn't plan on them lasting more than 90 minutes after opening (especially given that DCA closes at 8:00 pm).
If TSMM is your son's favorite, I would head straight there from rope drop. If you get there soon enough, you could probably get in a couple of rides without a substantial wait time. After that, I would head back to get a RSR FP. You'll probably only get one RSR FP if you go this route, so if you wanted a second ride, you'll have wait in the standby or single rider line.
If multiple RSR rides is your goal, then get a RSR FP when the park opens (be in the front of the rope drop crowds on the left side). Then after you get your FP, head to TSMM and ride that. Hopefully the RSR FP line moves quickly and you get to TSMM before the line builds up. Then as soon as the window opens for the RSR FP, go back and get another one.
The biggest wrinkle to this is if RSR is not operating at park opening. They do not distribute RSR FPs while the ride is down, so that could impact the schedule.