You'll need to add him to the room. They check Bands.
On a duplicate thread they posted they did have the DP and didn't want to pay for him for that. Someone else on that thread posted they were refused to add at the front desk because they had the DP. My experience has been they offered to cancel and rebook even with the DP but I am not comfortable with the FD doing that with points and the DP. Someone else posted recently they had no problem adding a day guest and I'm pretty sure that was with the DP.This is assuming that their room has excess occupancy available, and the OP isn't on the dining plan. If on the DP, adding him to the room will be quite expensive as everyone in the room has to have the dining plan.
If you can't add him to the room, then technically he isn't allowed to visit SAB or any other pool. Whether the CM's choose to enforce that is hit or miss, though SAB is by far the most thorough with the room band checks.
I tried to add my husband and son to our reservation at BLT in November. My daughter and I had the dining plan and the front desk said that we could not add them since we had the dining plan.