Scissors is the easiest way to do it imo but you can fix this with clippers. Apologies, as I'm not good at describing but I'll do my best. What you want to do os find the widest part of his head and hold the comb flat against it so that it created a line parallel to the head. Comb the hair into it, keeping it vertical and parallel to the head. You should see a little corner of hair poke out where your increase in length is. You want to go up the comb with the clipper and even that out.

I hope that makes sense, if not DM me and I can add you on some sort of social media, find a mannequin head and show you. Or I can find you a YouTube video that explains it better than I do