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I'm happy that you have asked me to assist you with your child and hope that you can benefit from my many years of helping parents like you.
Benadryl is generally safe at any age when given in the proper dose. Unfortunately, he is going to be miserable with a stuffy nose regardless but it can't hurt.
Here is a dose chart based on weight for you to refer to:
Be aware that benadryl sometimes has what is called a parodoxical effect on babies, sometimes making them hyper instead of sleepy.
Benadryl is safe so it's worth a try.
Please let me know if you have any further questions.