Spinach — Food safety facts
Safe to share · Dogs · full breakdown on the linked food page
Small amounts are fine; large amounts of oxalates can be hard on the kidneys over time.
Short answer
Yes, that's fine. Spinach is safe for dogs in small amounts. Just avoid feeding large quantities regularly, as the oxalates could stress his kidneys over time. Offer him some plain water and keep an eye on his appetite and energy levels.