Probability of Sole/legal custody

We separated last Oct due to me finding out STBX was addicted to pain killers. My son has been living w/ me full time.
Then 2 months ago he gets fired from his job for embezzlement (he was a CFO of a company) and then I find out several
days after that he enters an inpaitient rehab facility for alcohol abuse for a month. He is now out and finally got a lawyer. His lawyer sends paperwork to mine saying he wants every other weekend and 1 day a week. Is this really feasable he could get this? In the past 6 months he has only been alone w/ our son 2 times for a max of 2 hours and I made him take a drug test prior (he babysat for me at my house). This was before he got fired and went to rehab. I want sole primary an legal custody.

Yes, it is feasible he could be awarded such a schedule. Sole custody is rarely awarded as the court seeks to provide for the best interests of the children which presumably, includes spending time with each parent. I suggest you file suit for custody and seek that your ex only be awarded visitation if the same is supervised.

So what is the difference between Sole custody and Primary physical Custody?

Sole custody allows the sole custodian to make all decisions for the child without input or consultation from the other parent. Primary physical custody means just that, the primary custodian has the majority of the time with the child.