My baby just turned 3 months. When can he start to eat baby food?

6 months. Years ago, breastfeading was frowned on & many made "formula" from boiled cow milk&karo. Since cow milk is designed for calves, infants were fed early & often to make up for deficiences in cow milk. Now that breast is back, or formulas mimic breast, solids are optional. The sitting child is more capable of participating & few sit before 6m. Babies can wait past a year and thrive on breast alone.
Start solids at... ...About 6 months. As far as baby food is concerned, you don't need to use it...Ever. Baby food is nice, convenient because it comes in a jar and you can take it with you, but it's unnecessary. All you have to do is mash or puree what every food you are preparing for yourself, and use that. Make sure it's smooth enough not to cause choking. Fresh food is healthier and tastier than jarred.