As you noticed, there’s lots of nonsense out there about gout and food. And the trustworthy information one manages to find is too often quite vague. There are some things reasonable people disagree about but there are a few things we do know about gout and food. If you really want to, I’d be willing to discuss that with you, tell you where you can find reliable information and so forth.
But if I may… in my opinion, it would probably be a waste of time. Indeed, the most important thing you said was that your boyfriend takes allopurinol daily. That changes everything. Getting allopurinol right is more important than getting food right!
Most people can easily control their uric acid by taking allopurinol regardless of what they eat… on one condition: they need to take enough!
If you think your boyfriend still has too much uric acid, maybe he should be taking more allopurinol. In some cases, it’s more complicated but in many cases the only thing that needs changing in order to fix gout is the daily dose of allopurinol. The typical dose isn’t appropriate for everyone.
A warning though: allopurinol is a serious drug. Your boyfriend should be monitored for side effects and the dose shouldn’t be changed without discussing it with a doctor.
Something else about allopurinol: it’s an old drug and you can stock it for years. It should therefore be quite cheap.
If you’re paying a lot for it, it may be because you’re paying for a brand. If for some reason there is no cheap brand of allopurinol for sale in your locale, perhaps you could plan a short holiday some place where you can buy a personal supply of the stuff cheaply.
Finally, if you want to experiment with your diet and measure your uric acid, that’s great! I’d be interested to know the results of your experiment.
But your experiment wouldn’t help your boyfriend much because taking allopurinol changes the effect food has on your uric acid. Everyone is different anyway but your experiment would be more likely to help someone who wants to lower their uric acid without drugs.