Most babies are ready to start solids between 5 and 6 months, once they can hold their head steady and show interest in food. It sounds like your baby is close to that stage, and occasional spit-up while rolling or after feeds is common and not a concern on its own.
Next Steps
You can start with a single smooth puree once a day, and let your pediatrician guide the exact quantities, water intake, and how fast to move to thicker textures based on his weight gain.
Health Tips
Please see your pediatrician promptly if he starts vomiting forcefully, feeding poorly, or not gaining weight as expected.