Wet wipes Flush able ones. For the soreness I would use AD ointment
Talk to him which I am sure you have just let him know he needs to take the time to make sure he wipes good or it will continue to hurt.
I have both boy and girls and I felt my son was always in a big hurry to wipe properly and had same issues.
I'd get a box of wet wipes and put them in the bathroom by the toilet. Some kids just dislike the dry feeling of the toilet paper and avoid using it.
cream or vaseline will help also maybe wet wipes ...buy soft paper to wipe .this is very common children dont always wipe well ... dont get on his case about it, makes the issue worse ...