Pandora, that's a worry for you. I hope that it is nothing serious, and that the weight drop is just from him becoming mobile. We had issues with dd4 not putting on weight in her first and second years - it took a long time for her to catch up but essentially she's been fine. (They did label it failure to thrive, and she's seen a paediatrician every six months for the last seven years. She was finally discharged last year, even though she's still technically very underweight.)