This may sound indelicate, but this is the very reason for which the breeches buoy was invented. Basically it was a canvas chair with long tubes for one's legs. The landlubber was sat in this thing and hoisted by a yard and swung aboard. Provided you have adequate means for receiving a person boarding like this, you could easily fabricate a carrying apparatus, something like a bo'su'n's chair but able to come apart, to be fitted to a person who is still seated when you fit it. Make the connection points sturdy, foolproof, and safe.
The person needs to have a PFD on before being lifted. A tether harness to retrieve him, should he go in, is essential. Have one person do nothing but tend this as he swings aboard.
You can employ a whisker pole or a spinnaker pole as a gin pole for the boarding-- I would fit either with its own halyard/topping lift and some arrangement for preventers or downhaul tackle(s) to position it properly. The pole lift can be led to a self-tailing winch. Just be sure to rig it so that the person coming aboard lands where you want him to be on the boat. For this the main boom may be your best choice. Detach the sheetline (having a snap shackle on it is useful now) and be able to swing it wide overboard. You'll want to fit an additional boom bail from which you can hang the lifting tackle so it's as far out as you can get it but still in a good place when the person swings aboard. The lifting tackle should be entirely separate from the topping lift, so that the boom can remain level without being encumbered by the lifting tackle. The topping lift needs to be stronger than usual to handle the weight of a person-- most topping lifts are deplorably too light; but this is easily done if you have a proper (halyard-type) topping lift. Also, the lifting tackle needs to be able to be tailed from either the dock or the boat-- many mainsheet lines are too short for this. It's vital that the person being ferried aboard doesn't feel anxious about the line tenders' possibly letting go, so having a secure location for both the sending and receiving parties is a vital part of the procedure.
I am a big proponent of using the boom as a lifting derrick and have done it on numerous occasions. I am currently devising a way to lift the outboard motor off the transom bracket and over to the dock using the spinnaker pole slung under the boom; but so far I have too many lines to tend by myself. Give me time on this!