You can practise a routine with him, as a fantasy journey or in role play, for example: «Get up, turn round, take your jacket off the hook» or «Whenever I leave a room, I look back at my seat and ask myself: Have I packed everything or is there still something there?» It is not helpful to call your son a «scatterbrain» or scold him for forgetting. Instead, look out for moments when he has remembered his things and ask how he managed to do this. Last but not least: Don't buy expensive things and practise patience and composure, it's easy on the nerves.
Hm, I'm a scatterbrain too. And nothing my parents said could cure me. I learnt to be a little less scatterbrained when I stood in front of a locked door at home, couldn't do gymnastics and lost my nice new denim jacket. So don't hold everything against your son, he needs to learn that his scatterbrained behaviour has unpleasant consequences. It won't cure him, but it will certainly make him better. I promise.
Ritalin? What you describe are typical symptoms of attention deficit disorder. Or no Ritalin. It could also be a not entirely uncommon scatterbrain problem, as can be observed in many eight-year-olds. The problem is that it is almost impossible to make a reliable differential diagnosis from a distance. But not necessarily from close up either. So it is up to you to decide under which diagnostic label you want to view your son. Until you are ready, I would continue to try to remind your son of all sorts of things every day or every hour with the tongues of angels.