Brief Answer:
Examination and investigations

Detailed Answer:
Hi.
Thanks for your query and an elucidate history.

The symptoms you have described fit well into a condition called stricture (narrowing) of the urethra in the prostatic part or just after it.

I would advise such a patient the following:

Get a Per-rectal examination done by a General Surgeon or an Urologist to determine the situation of the prostate- its size, consistency, nodularity, tenderness on palpation etc; as also to palpate the urethra in its full length to see for any nodularity.
Another step would be to get retrograde urethrography done in which a XXXXXXX is passed from the external opening of the urethra and X-rays are taken.
Then can be subjected to direct visualization by doing Cysto-urethroscopy. This is diagnostic as well as sometimes curative as the stricture can be widened by cutting through it.

In addition to undergo all the other tests: Blood- complete blood picture, kidney functions tests, blood sugar fasting and post-lunch;
Urine- routine,microscopy, culture and sensitivity ( before start of antibiotic)
Ultrasonography of abdomen.

All these tests will help to get a proper diagnosis and give the plan for further management.

The symptoms you are getting is probably due to obstruction in the urethra acting as a valve when subjected to more pressure. When the local edema increases you would get the symptoms. The retrograde spread of infection into the spermatic cord and testes gives pain and discomfort as also in the bladder. Due to inflammation the local temperature is increased to give loose testes.

Since the erection is a process which is started by thought, the pain as you described would never allow the full erection.

I hope this answer helps you to get a systematic outline to go ahead with investigations, getting a diagnosis and a plan for the treatment.

Please feel free to ask further if you need any clarification; you may please rate this answer.

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