The short sleeve and leg rompers over a nappy will be fine, as long as they are cotton and loose fitting. Sun hat when in the sling, a nice cotton liner for the pushchair with a good UV sunshade. I think he is most likely to get too hot while in the sling and against your hot body! Also any arms and legs sticking out are at risk of burning.
While you're on the beach, you can cool him off by popping him in one of those tiny inflatable toddlers boats with some seawater in it (under shade of course)
Indoors anything that is loose and cotton. Was easier for me with DD as she lived in simple cotton dresses, but for a boy I think simple cotton T Shirts, I wouldn't even bother with shorts - his nappy will be fine.
Whatever you choose, just make sure its not too fussy or tight. You can always add a cardi if he gets cold, better that then overheat.
I don't know how long you are going for, but if it's any longer than a couple of weeks you may wish to cobble together a cotton sleeping bag for him, I did that for DD and it was a godsend.
We will be spending a reasonable amount of time in Cyprus soon, with DS who will be almost 3 months by then. It's already 32 degrees (hurrah!) but I'm now struggling to know what clothes will be suitable. He's in the sling a lot, so can get quite hot in that anyway. On the beach he'll be in the sun tent of course, but dressed in what?! I'm thinking just a nappy most of the time? Maybe with cotton shorts?
Ah if only we had this kind of problem in the UK ;)