That is exactly what I do !
Top Marks to you ..
Tax the car on line as you say - do the new V5 on line in his name - he'll get that back with in 2/3 Days , then put the plate on line - he doesn't even have to visit you - do it over the phone if he gives you new V5 number and numbers off his retention form ! #simples #quick #easy
If taking it off the car -put new v5 in his name - leave old V5 intact in his name ( Do not put into trade ) - again when new V5 arrives as as long as both V5 are same post code do it on line
Note: If the car you're selling him has not been taxed for 3 months or over then it won't always let you transfer the plate , you'll need to send all documents to DVLA , but to be fair they come back within days!
Hope it helps and I'l explained it clearly !