6 months old started eating solids and now he is totally refusing formula milk. what can i do to make him take his milk?

Change the sequence: Only allow solids when & if he takes what you consider an adequate amount of formula. When you want more of a behavior you reward it with whatever he considers of value. Solids are not necessary to normal growth & development of infants until well past the first year. Limiting the total volume to a few tablespoons or less is enough to begin the transition to table foods.