time4learning.com has a fairly robust section on measurement (it covers capacity, temperature, weight, and length separately). It discusses the tools needed for each, and goes over the metric and American units. You can start him at whatever grade level you think is appropriate (like, even 1st grade level, if need be; he'll finish it up in a couple of days -- or even one day, if he's so inclined) then, move him on to 2nd grade level, that'll take another coupe of days, then go on to 3rd grade, etc.
You should at least look at the demos for time4learning to see if its the type of thing that would tickle him. You get a two-week money-back gurantee, and, assuming the program appeals to him, I have NO doubt that he would be up to grade level -- or beyond -- in measurement during that two-week period if you just let him go to town. Now, nothing substitutes real-life measurement, I think, but this is a pretty painless way to help him out (plus there *might* be some other aspects of the site he could take advantage of those couple of weeks).
BTW, you can't just "choose" the measurement section, or even the math section -- you get all or nothing -- but there's nothing to prevent you from using the site the way you want.
