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I don't think you adding him as an AU would be very beneficial.
There are a couple of things he can do to raise his score, in order of importance:
1 - Contact the lender with which he received the late payment and ask for forgiveness. This is known as a goodwill adjustment request. He can try by phone first, if that doesn't work start sending letters/emails. If they removed the late payment and that's what caused his score to drop X Points, he'll immediately get back those X points.
2 - Lower his utilization. While 30% isn't terrible, getting it below 28.9% will result in a score gain and getting it below 8.9% will result in an even greater score gain.
3 - You mentioned "95%" of on-time payments. That sounds like a number provided by Credit Karma "fluff" software or something similar. Understand when it comes to on-time payments, there's 100% (like yourself) and then "everything else." Anything below 100% or less than perfect generally means a dirty file instead of a clean one. Whether that's 99% on-time payments, 95% or 70% really isn't too relevant to scoring. Less than perfect = bad. Also, if you are using CK to see this number, it is only factoring in the last 2 years of payment history, where FICO scoring looks at 7 years. I he had additional late payments 2-7 years ago, those are adversely impacting his score as well but the fluff software isn't showing you that. If he has other negative items, reverting back to my Point 1 is likely his best bet to attempt to get them cleaned up.
That's what I'd recommend for him. For you, while off topic to the conversation, I'd strongly recommend getting your "high" utilization in check as soon as you possibly can. If you were able to get it down to at least below 28.9%, it may be worth considering adding him as an AU.