What Does A Lump Oozing Purplish Blood With Puss Inside The Arm Of A Child Suggest?

My 7yr old son has a hard marble sized lump on the inside of his arm a couple of inches from his wrist. It doesn t hurt him to push down on it but is is tender if squeezed. I poked it today and instantly purplish blood with some pus mixed in exploded out. Within an hour it was again very hard and marble shaped. What could this be