How likely is my son to become diabetic

Type I diabetes is an inherited problem. Type I sufferers find out, usually quite young, that they cannot manufacture enough insulin for their body's needs. They need injected insulin supplements for the rest of their lives.

Type II diabetes usually appears later in life. There is some genetic tendency, but it is brought on most often by poor choices in eating and exercise. It's linked to obesity. Sufferers become resistant to their body's insulin, and require more and more of it to function. At first the shortfall can be addressed with oral medications that either lower blood glucose, reduce insulin resistance, or stimulate more pancreatic insulin output.

Source.
http://wiki.answers.com/Q/What_is_the_d ... I_diabetes

You wont have any control over whether your son gets T1 diabetes and the figures in Australia put the incidence at about 1 in a 1000.

As for T2 you can only teach your son good habits in the hope of never getting or delaying onset. There would be some genetic component to getting T2 but poor lifestyle factors can also increase those risks. Keep fit and well.

Quick answer, you can only do your best, Type 2 has a genetic basis but lifestyle in the form of diet and exercise is a big factor too.
Long answer
Type 2 has a strong genetic basis, there are specific genes (and more being found all the time) that predispose a person to developing it but It seems that the environment plays a big part in whether diabetes develops or not.
There are groups of people who do not get type 2 whilst living a traditional lifestyle, but when their lifestyle becomes more westernised diabetes becomes more common.
(think about the huge rise in diabetes in Asia)
The Pima Indians are one such group, those living in the US have a 50% probability of developing type 2 before their 60s, . They have been extensively studied since the 1960s in an attempt to find what causes such high rates of diabetes. One finding was a gene that seemed to contribute to the development of insulin resistance. This gene may cause a greater absorption of fatty acids from food. This would not create a problem with the native diet (which was fairly low fat) and could be a benefit with fats being stored in the body when available for times of famine. With a Western diet, with readily availalble fat the excess may contribute to the development of insulin resistance. http://diabetes.niddk.nih.gov/dm/pubs/pima/index.htm

Given that there is this interraction between genetic inheritance and environment its difficult to know how accurate any inheritance figures are but these are those given by the American Diabetes association for type 2.

In general, if you have type 2 diabetes, the risk of your child getting diabetes is 1 in 7 if you were diagnosed before age 50 and 1 in 13 if you were diagnosed after age 50.

Some scientists believe that a child's risk is greater when the parent with type 2 diabetes is the mother. If both you and your partner have type 2 diabetes, your child's risk is about 1 in 2

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For comparison the figures for type 1 (which incidently can occur at any age and may be more common in adulthood than in childhood*.)

In general, if you are a man with type 1 diabetes, the odds of your child getting diabetes are 1 in 17. If you are a woman with type 1 diabetes and your child was born before you were 25, your child's risk is 1 in 25; if your child was born after you turned 25, your child's risk is 1 in 100.

Your child's risk is doubled if you developed diabetes before age 11. If both you and your partner have type 1 diabetes, the risk is between 1 in 10 and 1 in 4.

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There is also a relatively uncommon type of diabetes called MODY,This is very strongly genetic with a 1 in 2 risk of a child developing it. There are several types, some of which are comparatively 'mild' and each type is asscociated with a change in a single gene. As research on it is quite recent it may only be found that a family has this this form of diabetes when a young person in the family is tested for it. (parents and grandparents were more probably originally diagnosed as type 2) http://projects.exeter.ac.uk/diabetesgenes/mody/hnf1a.htm

*' 'adult-onset type 1 autoimmune diabetes is two to three times more common than classic childhood-onset autoimmune diabetes'
Type 1 Diabetes in Adults: Principles and Practice [Hardcover]
Serge Jabbour (Editor), Elizabeth A. Stephens (Editor)