It sounds like he is having enough feeds anyway (even if it's less than you are used to). If he is happy to have water, I would just focus on DH giving him plenty of water during the day (especially while the weather is hot) & you can just feed him before/after work - sooo much easier than expressing too

If you remember, I had dropped DS to 2 feeds a day by 11 months & he's been just fine. He drinks lots of water (two big bottles a day) - plus a small amount of cow's milk (50-100ml?), some yoghurt & cheese.