Blow job

Hi ladies my husband and I have been married 4 years. I have asked him numerous times if he would like me to give him a blow job as I think I'll enjoy it and obviously he would too. He says that I'm his wife and he doesn't want me to do that I think it's sweet that he's trying to respect me but I think it will spice Things up. I'm sure his past gfs have given him BJ's. Any advise on how to get him to be comfortable with it?