Translation:The red pencils belong to Andrew.
"The red pencils are Andrew's" should be the default solution since it is both correct and closest to the original.
"lui" is mandatory. "Ale" is just a preposition. "Ale mele / ale tale / ale lui / ale ei / ale noastre / ale voastre / ale lor" = mine / yours / his / hers / ours / yours / theirs.