We wound up taking ours through Walks tours and did the VIP Colosseum
Night Tour with Underground & Arena.
The Walks of Italy website shows this tour as currently unavailable as is their "VIP Colosseum Underground Tour with Roman Forum & Palatine Hill" tour.
jdarrellsterbak, the issue is that even 'legitimate' 3rd party tour sellers cannot always get their hands on underground tour tickets. They sell them in hopes that they can but Rome forums are fuil of reports from individuals whom have had their tours cancelled before their trips - and sometimes at the last minute - because the company they purchased from couldn't get them. Unless you're buying from the Colosseum's own site, there's no foolproof way I know of to get access to the thing.
What I'd recommend you do is to assure your husband that he can see a good share of the underground from above, either from the arena floor, should you attempt to purchase tickets which include that part, or tiers open to visits with regular entry tickets. Go for the arena floor as it'll get you a bit closer to viewing what's below.
