I suspect the last tour might have convinced him that Green Day would be a great place to hang his hat for the long haul. He's become more than just a touring musician over the years, and the band sounds at their best when he's in the mix with them -- Billie Joe is a very good guitarist, don't get me wrong, but his real strengths are his lyrics and now especially his vocals. Jason ups the ante because I think his talent on the guitar surpasses Billie's. That's not a bad thing at all, and I think it contributes so much to Green Day's synergy -- that is, where the whole band is greater than the sum of any individual part.
If Jason came to realize this on the last tour, and the rest of the guys renewed their offer to him of an official spot on the marquee, it wouldn't surprise me if he accepted this time.