If my son drives a vehicle in my name only and has an accident which injures someone, in addition to myself can my husband also be sued? 

There is some risk. First, from  what you write, I assume that your new husband is not your son's father; if he is, and if your son is minor, then your husband would, of course, be very liable, as the parent.

In theory, if the vehicle is registered in your name only and was purchased with non-marital assets, only your personal assets should be at stake. Where the risk comes from is: 1) joint assets can be targetted, and at the least encumbered--therefore a lien could be put on a marital home; a joint bank account frozen, etc. 2) Judgment creditors may not understand or agree that certain assets are only his, and not yours, or believe they were transfered to you only to hide them from creditors (in defraud of creditors), forcing your husband to prove they are  his alone.

Of course, with a million dollar policy, the odds of any lawsuit exceeding that amount, let alone by enough of a margin to be an issue, are very small, so it may be that the risk is also essentially theoretical. But as stated, when two people are married, if one is sued, it is almost impossible to stop there from being at least some collataral impact on the other. Whether a small risk--or rather a small risk even assuming in the first place there is an accident causing significant injury, which itself is highly unlikely--of collateral impact is enough to be of a concern is something you and your new husband must answer for yourself.

Note that if you son is an  adult, even if he can't afford his own car, you may wish to give him the money for a car and pay his insurance premiums--that will sidestep the entire issue.