DS is now eating solids a few times a day and his poops have increased to about 3-4 times a day. Is this normal?

When he was exclusively on formula, he pooped once a day.

Also, will it slow down once his body gets used to the solids?

Very normal, happened to my DD too. She would go maybe once a day when on formula and suddenly it was 3 and 4 when we started solids. I think she is still around 2-3 per day now that she has adjusted to solid foods for several months now. But it used to be predictable, after she ate, she would go. Now you never know when, usually when we're in the car headed out somewhere, ha ha.