As a former 4th generation Seventh Day Adventist and currently the only Apostolic in my entire family that worships on Sunday, I personally found the main oppositions you will come in contact with is the eating of pork and shell fish, the soul sleep doctrine, the seventh day saturday doctrine and the trinitarian doctrine coupled with the baptism in Jesus Name doctrine that we oneness believers teach. I would show him scriptures that refute the keeping of ceremonial laws, the reason why we observe Sunday instead of Saturday, show them scriptures concerning what happens after death, and finally the
Acts 2:38 plan for ones life in its totality. The Holy Ghost is then going to have to do the rest as this is how I transitioned from the Sabbatarian doctrine to the Apostolic Faith. It also helps to show the similarities between the two faiths like the holiness standards both denominations teach. Hope that helps a little.