I don't really have much advice, since J and I are just starting our first real poly relationship..a triad and have no experience in a quad.. But I wish you all the best of luck in working it out, if Stew really does want this. Sounds like he is just having a lot of jealousy issues that he needs to work on which is very normal.
There's some great threads on here and on the net on dealing with jealousy that really benefited me in the past and I refer to whenever I'm feeling a little insecure.
Have you or Jenn asked him why he's behaving this way? I know he said it's because of emotional bursts he's having here and there, which do happen, but I'm thiking he really feels that you and DH are a threat to him... that perhaps Jenn will have more excitement and fun with you two, together and separately and that makes him feel inadequate.
There was a great guide I found somewhere online, that I had printed out for J and I.. if I find it, I'll PM you with it. It might help Stew a bit. If he is serious about continuing in this quad and wanting to make it work, he's going to need to address his insecurities and conquer them. It sounds like the rest of you definitely have a handle on things and are comfortable with each other and it's progressing, aside from Stew's behavior.
Bi-sexual poly female dating J (straight, poly male)