Right now your boyfriend is trying to help the best he can by getting you to stop worrying so much. Worrying won't help Diffy, it will just hurt *you*. There are many things that you can do to help Diffy once he comes home.
Make sure the water he drinks is pure, filtered water. Try to filter out chlorine at least. Wash his water bowl daily and make sure he never runs out of fresh water.
Check into his diet. Many commercial cat foods, well, to put it politely, are really bad. Many times even your vet won't tell you (because most don't know themselves!) just what goes into most commercial animal food. Go through and read some of the posts in this forum concerning diet and food. There are some good healthy prepared foods out there, but some people much prefer to just switch their pet's diet over to raw. My cat absolutely loves raw chicken and hamburger meat, once he got accustomed to it. Took him a few offerings where he snubbed it, but eventually he discovered he adores it. I still offer kibble (simply because he demands to *see* available food - rescue kitty with some interesting personality quirks), but supplement with raw at least 4 times a week plus add enzyme and essential fats supplements to his food the other days.
Raw has an added benefit of being one of the suggested remedies for FLUTD (Feline Lower Urinary Tract Disease)!
Here's a good site to read up on some other ways you can help Diffy. Some herbs they suggest are couch grass root, birch leaves, nettles, goldenseal, and flax seed. Others to check into are corn silk and slippery elm to help soothe the irritated urinary tract and uva ursi a good general antiseptic that is especially effective for cystitis or urethritis. Check at your local pet super store or natural health or herb store for specific feline formulas containing these herbs.
The *best* thing you can do for Diffy right now is to calm down and stop worrying so much! Diffy can come through this fine and with what you now know about how to avoid the crystal buildup again, you can both avoid it in the future or at least treat it MUCH sooner so it never gets this bad again. Our furry four footed companions are very energetically attuned to us and if you are sending out panic and worry then he will pick up on that. For you and for him, listen to your boyfriend, calm down, and believe that everything is going to be ok. If you believe it, Diffy will believe you and believe it too. =)
Good luck and give Diffy some extra kitty scritching from me. =)