We used to have the same problem when Tyler saw different doctors. We got all kinds of different opinions on things, so in the end, I just weighed it all and went with instinct. With your DS, you know him best...weight and percentiles are only one of many factors to consider in the decision to start solids. And babies can be all over the charts from visit to visit without it meaning a thing. Tyler's always been above 100% on height and started out 80% for weight. Since about 4 mo, his height's stayed the same but his weight has been around 50%. And it's no surprise...DH and I are both built that way. So it's very possible that your DS is just balancing out. And even if his weight "loss" was a concern, starting solids isn't necessarily the answer. Ounce for ounce, breastmilk has far more calories than any solid food.
In the end, I'd say go with your instinct!