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First, get married BEFORE you get a mortgage together, and really you should get married before adding your soon to be DH as an AU or joint holder. Seen a couple of folks report a relationship going in the toilet BEFORE wedded bliss, and one party getting stuck with a bunch of the other's debt.
AU status is still helping him, if they are reporting. Has he pulled his CRs to see for himself? If not, have him pull them, FOC, at annualcreditport.com
I know from personal experience that Wells Fargo tends to be easy to convert to joint status, at least for a spouse. Don't know if they get all preachy or anything for folks shacking up. I don't believe AMEX does joint accounts, at least not anymore, but give 'em a call and ask. Chase might or might not do joint. But do ask.
Lenders are fairly hungry to make loans, and if you have good credit and he has a "thin file", then it's likely y'all would get a good deal.
If his PIF truck is of recent vintage, he could get a loan on it, stick the money in savings, let the loan report, add a good TL to his CRs, then PIF the truck loan. He'd only incur the cost of the loan papers, usually around $10, and the month or two of interest.