Have him pull it toward the floor while flaccid count to 5 then let go, count to 5 again then measure the length from the base to just behind the head, do not include the head.

Diameter

Measure the area around the shaft just behind the head, you want this measurement snug but not tight. Use a cloth tape some where near 4" will be the number you come up with divide this by 3.14 and that is the diameter for the shaft size. Using your cloth tape measure around everything at the base of his shaft and testie's. Pull the tape really snug where you can only slip one finger in between the tape and his body. Some where around 6 1/4" will be the measurement you are looking for divide this by 3.14 and this will be the testie ring size you need. Let him do the measuring if you do it he will try to get excited and you might be a long time getting the measurements right.

Another more accurate way to measure and you want to make sure you get everything exactly right and get the proper size. I suggest you go to the hardware store and get a 2" pvc coupler cut off about 3/8" inch off the end of it. Try it on, take it back off and cut a section out of the top, squeeze it together you may have to apply some heat to get it to push together, Repeat this until you have a ring that is right, you will want a ring you cannot put more then one finger in between your body and the ring. Once you determine the correct size measure across the ring making sure the ring is round, you may need to once again heat the ring up to push it into a circle. Do the same thing with a 1 1/4" pvc coupler to get the shaft size right.