Everyone who's ever had to break up with someone they cared for has wished there were an easy way out. Unfortunately there's not. Any attempts to spare his feelings by trying to let things just peter out rather than being clear about where you're at will backfire and cause more pain overall, not less. Your best bet is probably just to tell him exactly what you've told us -- that you don't see this as a forever relationship, that you care for him and would be happy to date him causally or have an FWB relationship, but that you're ready to date others. Be strong, for both of your sakes.
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.