My ds1 has the same problem with suncreams and we still haven't cracked it, either. Last year, he was OK with Soltan Once, this year he came up in a pinprick rash all over when we put it on. We have occasionally found suncreams that he only reacts mildly to, following which his skin seems to settle down a bit and tolerate them, but the next year he reacts more vigorously to that type of suncream and we have to start searching again... Most of the time I just avoid putting the stuff on - he's got an SPF 50 legionnaire's sunhat and tends to stay in long sleeves until it's much too hot to be bearable! He also has body-covering swimsuits, to avoid his torso being exposed... Part of me is hoping that as he gets older, his skin will get tougher and less reactive (not that he reacts to anything other than suncream), as I'm not sure he'd appreciate the current covering up approach forever (he's only 5 at the moment). However, there is a lot to be said for covering up in the sun, rather than plastering chemicals all over your body that react with the sun on your skin in ways that are not 100% proven to be harmless themselves, even in people who don't have an immediate physical reaction to them.
One thing I have been told is that lots of people react to PABA (para-amino benzoic acid) in suncreams, so you should ensure they are PABA free. I have a feeling, though, that most sunscreens no longer have this in, anyway, although some still have Padimate O or octyl dimethyl PABA in, which are PABA esters and also provoke allergic reactions in a fair number of people, so avoid these, too! Basically, I think the general advice is that you should try to find a sun cream where titanium dioxide (or zinc oxide) is the main active ingredient, as this is an effective block against UVA and UVB and doesn't tend to provoke allergic reactions in the way many other chemicals often used in suncreams do. Oh, and one other thing - I've read somewhere that cinnamates in suncreams are also somewhat more prone to provoke reactions in people with sensitive skin that other types of UVB absorbers.
And finally, please tell me if you do find a good suncream, with high UVA and UVB protection, that your ds doesn't react to, as I'd love to save my money and just get one I'm fairly certain will work on my ds!