I am Dr. Arun and will be helping you today.
I am sorry that your question was unanswered for long.
The decrease in size can be due to;
1) Hormonal issues
2) Inadequate filling of blood in penis tissue.
He may need following investigations:
1) Testosterone levels
2) LH estimation
3) Thyroid function tests
4) duplex ultrasonography
5) Ultrasonography of testes
7) penile nerve test,
8) snap and strain guage test,
9) penile biothesiometry.
Not all of these investigations are done at the same time. First a blood work for the hormones would be done. Later if needed, Duplex ultrasound and other may be advised.
Please feel free for your follow up questions.
I would be happy to assist you further, if you need any more information.
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