Do you know when he's going to go? Is there indication (there usually is) or a schedule? I found that when my daughter was about that age she was having a similar problem (holding it in for so long and then it being huge and messy, it started when she started eating solids, she was breastfed too and didn't really eat solids until 7 or 8 months) and I started putting her on the potty every morning, and after she was 8 months old I never changed a poopy diaper again! It just helped her being in that position and having the schedule. Even if that's not the reason, it's much easier to clean up. With my younger daughter I did the same thing, but she goes much more often, but I only clean a poopy diaper if we're on a weird schedule.
Anyway, it might work for you, it did for us. We used the little baby bjorn potties and they loved them.
Congrats on breastfeeding for so long, it's one of the best things you can do for your child, and yourself and certainly helps their digestive tract.
Someone I noticed mentioned acidopholis, and I thought I'd say, I gave it to my baby because of thrush, and I'm not sure if it helped with digestive problems, but it certainly helps my older children, myself and my husband with digestion issues. Might be with a try!