Keep him hungry!
Some babies can have a bit of oral aversion, meaning if you introduce solid a bit late, your child will keep refusing it since he is not used to it.
You can keep him hungry a little (he should be easily able to go for 5-6 hours without his milk if he is healthy). Then start him with something he is more likely to eat first, like apple sauce or banana. Rarely you get referred to occupational therap.