Thank you for your question.
I am Dr. Su S. and I will try my best to help you.
As he 7 months old, please check whether he has taken any other food in between breast feeding.
Over feeding can be possible.
Give next feeding a bit late and small in amount.
Frequent small feeding should be tried now.
There can be a viral infection, other symptoms of viral infection may not be prominent in initial stage.
If the projectile vomiting persists in spite of taking precautionary measures, then you need to take him to ER.
Otherwise you can wait and watch.
I hope this information will be helpful.
If you have any query, please feel free to ask.
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Wish his healthy life.