How old is your nephew?
My granddaughter slept in her car seat in the house and usually didn't object being put into it until she started walking. Singing and toys helped when she did object. But mostly we just consistently persevered until she learned that it wasn't an option. I would then put a childrens CD on and this usually quieted her. If I remember correctly the times that she objected were when the seat was already in the car. We put her in the seat in the house and then carried her into the store in the seat. While in the store I entertained her by talking and singing and by giving her toys. She is now 6 and every once in awhile she wants to sit in the baby seat attached to the grocery cart. I think she has good feelings about those days.
It's important to stay firm and show no hesitancy about putting him in. I felt anxious because she was so adamant about not getting in and she picked up on my tension. I watched her mother put her in the seat and saw that she was all business. No singing, just a firm you have to be in your car seat. sometimes she'd say, it's the law. I want you to be safe; the reasons for the seat. Always in a firm voice. She did give her stuffed animals. Monet is now 6 and she still likes to have at least 2 small stuffed animals with her and she carries on a converstation with them acting out "stories."
I trained myself to show less concern and anxiety and getting her into the seat became easier.
If your nephew is still a baby you could try giving him a bottle when you put him in. This also helped with both grandchildren. It became a sippy cup and cheerios as they got older. It seemed to me that often I was in a rush to go and hadn't thought thru the possibility that they were hungry. Afterall I eat whenever I get a chance. lol I could wait until later when I was hungry. Little ones don't have the ability.
And sometimes I just had to let them cry it out. And without my trying to get them to stop. Again they'd pick up on my being anxious for them to stop. I would talk with them in a relaxed way. As they got older, the CD almost always worked. We listen to music and stories.
When Monet was 3 I was listening to a book on CD, The Life of Pi. She was fascinated by it and so I got to listen to something I really enjoyed. she still asks to hear that book again. Unfortunately it was from the library. But I will buy it when I find it on sale. And now that I remember I will check it out again.
Getting and keeping babies and children in car seats is a common difficulty. But with consistency they eventually learn that they have to be there and you are not going to respond to their cries. It is an opportunity for them to learn to accept rules, which are few at this age, and how to entertain themselves.