It's hard to tell how much they're getting when you're nursing, but your best bet is to keep an eye on his nursing sessions. If he starts to lose interest or is nursing for less time, than that usually means you might consider cutting back on his solids to help his nursing appetite. Another cue can be if he starts waking at night out of hunger. Believe it or not, breastmilk is actually much more calorie-rich than any solids, so if he isn't getting enough during the day it can affect his nights. Chances are, he won't dehydrate, and you'll know if he starts to. Signs of dehydration can include fewer wet diapers, constipation, no tears when crying, or you notice the urine in his diaper is a dark yellow.
Most beginning solids eaters eat anywhere from 1 tsp to 1 tbsp at a meal, but some eat more (or less) than others. Keep an eye on his nursings and go from there!