Dependency status does indeed make a difference.
If your son is your dependent, then you must include his entire 2016 income as part of the household MAGI. If he is not your dependent, then his income is a concern for his tax return, not yours.
Because you have shared a Marketplace plan, you do need to decide what your percentage of the Premium, SCLSP and Premium Tax Credit is. The percentage can be anything from 0-100%. If you want, you can claim 100%. The consequence is that if you paid too much, you will get the credit based upon your percentage, and if you paid too little, you will pay the extra according to your percentage.
You might want to try different percentages to see what gives you the best result. You may choose the one that has you paying the least tax.
