What math and spelling curriculum/books are you using? If you are happy with the manner in which they present the concepts, you may want to stay with that publisher and see if the publisher has placement tests like the Singapore ones Kriston mentioned.
Do you have a local homeschool used book store where you could go spend some time with him looking over various levels of books and seeing what seems to fit?
If you aren't married to one specific publisher/book series, how about working on etymology instead of straight spelling? He could learn the origins of words and how to spell word roots and put them together to form words.
For math, maybe just make a big leap and go up a few grade levels and see how he does. You can always have some back-up workbooks available to fill in the gaps in things like fractions, etc. Edward Zaccaro's math books are fun and even his Primary Grade Challenge Math one includes up to fairly advanced 4th grade concepts. Those may be a good supplement.
