@NJres , Life is not that simple. Even if you are elidgable for employer based insurance, you might still be eligable to buy on the exchange with a subsidy. I do not pretend to understand the rules. I am suggesting you look at the instructions for 8962. From 2015 IRS instructions for form 8962:
"Eligibility for minimum essential coverage. In most cases you are considered eligible for minimum essential coverage if the coverage is available to you, whether or not you enroll in it. However, special rules apply to certain types of minimum essential coverage as shown below.
Employer-sponsored coverage. Even if you and other members of your tax family had the opportunity to enroll in
coverage offered by your employer for 2015, you are considered eligible for employer-sponsored coverage for a month only if the offer of coverage met a minimum standard of affordability and provided a minimum level of benefits, referred to as “minimum value.” The coverage offered by your employer is generally considered affordable for you and the members of your tax family allowed to enroll in the coverage if your share of the annual premiums for self-only coverage is not more than 9.56% of your household income for 2015. "
So if I read this right, your son might be eligible for a subsidy even if the his employer offers a plan if it is not affordable based on the above. In his case, 12000*.0956/12 is $96.56 so the monthly cost of 135 is not considered affordable. However I am not a tax accountant so I could be totally wrong.
I wish that ACA was not so complicated! There are so many in and outs that trying to fully understand the rules is tough. Maybe, I am just too dense.