Baby losing weight at 5 months

Has anybody's baby lost weight at around 5 months? Our son over the past 3 weeks has lost 70grams rather than putting it on. He is mainly breastfed with a bit of vegetable puree in the evening. We have another doctors appointment at the end of the week but wondering if anyone else has gone through the same thing? I've tried giving him some formula also but have had no success in getting him to drink any. I've also tried expressing and topping him up with that but he won't take that either. When he feeds it's only for about 5 minutes and does that every 2 hours