The doctor told you that was venom still coming out of the wound? If so, Id find a new doctor. Thats not venom, not to mention the fact that rays do not inject you with venom. The sheath that surrounds the barb contains the toxin. When a ray stings some or all of the sheath comes off and stays in the wound and breaks down causing the pain.
If the wound still had any of the sheath in it, which it shouldnt if properly cleaned, his body would have long broken it down. What that is is simply puss from the infection. NOT venom.
I will give it to your son though, that is a NASTY wound. It makes my little sting look like a mosquito bite in comparison yet mine still hurt like a SOB!