Is it safe to use a neck pillow to support baby’s neck, whilst he is in his car seat? If I don’t use this, his chin rests on his chest.

How old is your baby?
What type of neck pillow are you referring to?
Have you seen our information on car seat safety?
Avoid long periods in the car with baby sleeping &/or in “chin on chest” position.
https://rednose.org.au/article/are-there-recommendations-for-car-seat-or-baby-seat-use

Red Nose also has this information about use of any form of head restraint (or pillow like product) in car capsules/seats.
Devices that restrict the movement of a baby or the baby’s head are not recommended

Avoid prolonged periods in car seats, strollers, swings and bouncers as this places additional pressure on the back of the head

https://rednose.org.au/article/babys-head-shape