Normal. Bf babies spit up less and since the milk digests so much easier the water is absorbed very fast - so spit up can be thicker.
Also if he ever has a massive spit up after eating and wants to eat again don't think you're empty - you are always making milk. It's a "problem" I've heard over and over in the bfing moms group and IRL at my LLL meetings - mom thinks she's empty and baby spit up so now he/she needs formula. Just put em back on the breast : )