Have you tried him on weetabix? My dc3 loves it and in the early days of weaning enjoyed it with stewed fruit or berries. Maybe an idea for evening supper? I find them naturally sweet tasting so he might take to them. I totally appreciate you want to move away from the sweet stuff, but for now if you manage to offer a wide a range as possible of all different flavours it might help.
Also rusks were another good one for dc3 (often frowned upon here for the sugar content, but he's dairy intolerant, so I tried anything that bumped up the calcium, including the reduced sugar rusks, as fortified!) he'd happily munch one or have it as a pud softened with oat/almond milk, again sometimes with stewed fruit mixed in.
Melon is a good one too (cantaloupe is a fave with all three here), and great in chunky fingers and soothing when teething. Also cucumber might then be a natural progression.
How is he on bananas? For some reason all three dcs took a few months to be able to tolerate bananas, they had upset tummy / trouble digesting them initially, but a great filler rather than just a fruit purée.
Dd was a slow starter food wise. Didn't really start eating properly till 8-9 months, just went at her own pace.