That's fantastic to hear that you're getting back into Warhammer 40k, especially with your son showing interest as well! Starting a new force with the Necrons sounds like an exciting endeavor. It's great that you have a classic marine army and now have the opportunity to explore a different faction.
Your Sons of Horus chapter, led by Horus Caphelos, 3rd Company Captain, sounds intriguing. The Sons of Horus have a rich lore within the Horus Heresy era, and it's wonderful that you've chosen them as the foundation for your new force. With their unique color scheme and iconography, your army will surely stand out on the battlefield.
It's always exciting to dive back into the hobby and immerse yourself in the Warhammer 40k universe. Enjoy painting your new models, crafting your army's story, and engaging in epic battles with your son. May the Emperor's blessings be with you on your new journey in the 41st millennium!