It sounds like a Wrist Ganglion Cyst. A wrist ganglion cyst is a swelling that usually occurs over the back of the hand or wrist. These are benign, fluid-filled capsules. If it is a ganglion cyst, it is normally not painful. Sometimes if you bump it against something hard, then it can hurt a little. Take him to the doctor, so that they can make a proper diagnosis. Goodluck
Yes it does sound like a ganglion cyst. You will have to have it checked to make sure. If it is a ganglion cyst, they can remove the fluid inside some don't come back but usually they keep coming back. You can have them surgically removed but it takes some time to recover (about 2-3 weeks) . They have to remove the "stalk" and depending on where the cyst is located indicates to the doctor how hard it is to remove and any complications that might occur. My husband has one on his wrist on the underside or by the radial artery so his is harder to remove because there is a nerve there and the artery. He is debating whether or not to get it removed surgically. Also, they will usually give a numbing agent like lidocaine and take a sample for biopsy which doesn't hurt. They can tell from the biopsy what it is.
take care and good luck,