Bismihi ta'ala

Irrespective of what maddhab he may be, it is extremely important to recognise and affirm good qualities for your potential spouse, especially in being honest and truthful.

It is permissible for a Shi'i to marry a Sunni, as long as he/she is able to preserve their Shi'i identity and practice their Shi'i ways without pressure or backlash. 

If he is interested in the Shi'i madhhab, then that will be his choice, but he needs to make it clear to you as to why he is saying he is Shi'i, but, might be doing certain things the Sunni way.

Is it because he is in fear of his life, and therefore he needs to do taqiyah? Is it due to lack of understanding, and what slight differences there are between the Shi'i method of praying and the 4 Sunni methods? 

Or, is it him trying to please you and make your hear what you wish to hear only, which means he is not be honest to you? 

This is a conversation that needs to happen.   

With prayers for your success.