Technical interview is OK, ask the guy about his job. Give him a coding problem in his language of expertise. Check his knowledge of tools, techniques and best practices in his area of expertise. Do all the tech routine.
But please don't go on and on with Math problems endlessly over weeks and months. Asking arcane and rare facts about stuff and then deciding his worth over his perceived inability to solve problems in math which don't have any connection with programming in the real world.
It's a bit of a tall (impossible?) order for a programmer - regardless of skill or experience - to expect to interview without going through some serious technical conversations and writing some code to solve problems.