Brief Answer:
Diabetes/ fertility

Detailed Answer:
Type 1 diabetes is not known to be a direct cause of male infertility.

There are 4 situations where there can be an indirect impact:

1 Anti sperm antibodies

Because type 1 diabetes is an autoimmune condition, there is a general possibility for other associated auto immune conditions to be present such as thyroid problems. Infertility due to antibodies destroying the sperms is a rare possibility.

(An autoimmune disease is one in which he body's own defenses attack cells/organs and cause problems)

2 Obesity
There are some papers published in reputed medical literature where severe obesity associated with type 1 diabetes resulted in low pituitary signal to the testicles and lower sperm production.

3 Autoimmune Hypophysitis causing Hypogonadotrophic Hypogonadism

What this big term means is that autoimmunity affects the pituitary signals (FSH, LH) to the testicles, and that results in poor sperm production along woth lower testosterone levels.

4 Autoimmune Primary Testicular Failure causing Hypergonadotrophic Hypogonadism

What this long jargon means is that autoimmunity affects the testicles directly and as a consequence of that the testicles can neither make testosterone nor sperm.

When I see someone like your husband in my practice, in addition to a thorough physical and genital examination I also order the following blood tests:

CBC
CMP
A1c
Urine Albumin to Creatinine Ratio
TSH
Thyroid auto antibodies
Lipids
Total Testosterone
SHBG (Sex Hormone Binding Globulin)

Semen analysis (formal lab test; not home kit)

Depending on the results of the above tests more may be necessary such as:
Prolactin
FSH
LH
Ferritin
Karyotype
MRI of the pituitary
Check his sense of smell

It would be best for your husband to see an Endocrinologist in person and consider a referral to Urologist as well.

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