The basic answer is yes, you can assume that you're half-siblings. That level of cM sharing only occurs for:
If you can safely reject the other types of relationships, that leaves only the half-sibling relationship.
As to further testing as George Gaál indicated, you can always try to test his (your) father.
If he's unwilling or unable, another option would be for both of you to test at Family Tree DNA and do the Y-DNA tests. You'll need to test directly with FTDNA, as the AncestryDNA test is of a different nature (autosomal test). If, in addition to the autosomal result you already have, you both are Y-DNA matches that proves that your common ancestor is on the male line. If you test up to 111 markers, and the genetic distance between you is 0, that will prove with 95% confidence that your common ancestor is within the last 5 generations.