A lot of factors can be in play that can potentially decrease sperm count. Diet is only one of those factors.

Aside from certain types of sexual dysfunction which affect male fertility here are some known factors that you can control with little effort:

-Excessive exercise. Studies have shown that exercising too much may lead to the release of too many steroid hormones. This can affect fertility.

Suggestion - moderate your exercise levels - consult a physician.

-Stress.

Suggestion - regular sleep, moderate exercise, regular meditation/relaxation, consult a physician.

-Obesity.

-Use of drugs, alcohol, or tobacco. Smoking tobacco, using drugs such as marijuana and cocaine, and taking steroids can reduce sperm counts.

-Exposure to toxins and environmental hazards. Pesticides, lead, radiation, radioactive substances, mercury, and heavy metals may affect fertility.

-Heat. Although the effect is usually temporary, high temperatures in the testicles could reduce sperm production. High heat could result from wearing clothing that's too tight and traps heat, frequent bike riding, or by taking too many hot baths or saunas.

Suggestion - let 'em breathe. ;)

Focusing specifically on diet, certain vitamin/mineral deficiencies can also impact sperm production. These include:

Zinc - number 1 with a bullet
Vitamin C
Vitamin B12
Vitamin E

Suggestion - sufficient quantities of each should be available in a /Men's/ daily vitamin such as Centrum for Men.

Disclaimer - I am not a doctor. You should always consult a doctor with medical questions.

Yes. Traditionally the people groups with the best fertility rates, with the healthiest babies, have used lots of seafoods in their diets. In Scandinavian/far northern countries it was high-vitamin cod liver oil; actual cod livers; sometimes fish roe. In other areas of the world, it was fish roe and other seafood, but oily fish have the vitamins and other related factors that can affect fertility of both men and women. Sometimes even inland groups made special efforts to import these foods, which were emphasized as foods for the newly-married (or those of that age).

Yeah you need to not do anything that is anyway bad i.e. smoke, check out the link below, also NHS website should tell you, or go see your doctor

Not sure if this book directly addresses sperm count but it is a really good book for, well, all things penis ( to put it bluntly ). Check out the Hardness Factor. It is a good book with information that is based on sound medical advice.