Congrats on the twins! I was able to breastfeed my twins for 15 months without needing to supplement with formula.
I can tell you right now, it does get easier! As for suggestions for pumping and then doing a bottle, I found it extremely difficult to find the time to pump, so I'm not so sure this would actually lighten the load.
My twins were my first babies, but I have twin mom friends who had toddlers first, and some of them also exclusively breastfed their twins. They had some ideas similar to other posts. One would have her toddler get her special toys, and then would have her sit at mommy's feet while she nursed. Another one would nurse in a bedroom and close the door with the toddler inside (and, of course, toys or activities to do). Then, all four of them would be in one room.
Having twins is a huge challenge, so to have a toddler at home in addition only adds to the balancing act. Just do your best to make sure your son receives enough attention and love from you (I know, easier said than done, right?) so that he doesn't always feel left out.
Good luck, and congratulations on your babies!