Blue or green urine can be caused by food coloring. It can also be the result of dyes used in medical tests performed on kidneys or bladder.
The pseudomonas aeruginosa bacterial infection can also cause your urine to turn blue, green, or even indigo purple.
In general, blue/green urine is rare and most likely connected to something in your diet.
You may need to take your child to see a doctor for further evaluation.
Your baby's weight is okay for his age. If the doctor says he is fine,there is no need to worry.
Try introducing different variety of food to him.
Also, check out:
Is the green poop due to improper feeding?