We had this situation two years ago, older DC at grammar, youngest missed qualification (failed appeal/review) at the secondary school. We encouraged DC to embrace the secondary school and they were doing very well, top in top sets

however approached me in Year 8 to say wanted another go at getting into grammar. So sat 13+ (did not prep just went on day, totally relaxed about it) and qualified. DC was very fortunate to get a place at allocation day. Another child got a place for Y9 but had to go to appeal. DC was happy at both schools, but does love being with their older sibling. They are very close/good friends and have been great support to each other during some hard times. However a few friendships made at their other school are still going strong - they talk, message and meet up regularly.
Would DC be just as happy now if still at the secondary, probably, but for us (personal medical reasons) it just made it much easier to have them at the same school.
Good luck to your DS2, I am sure he will do well - probably be at the top in his new school, a definite boost. If you do decide to go down 12+ 13+ route just bear in mind, even if they qualify, there is no guarantee of a grammar place.