My Domestic Partner and his Son have been on my Medical/Dental and Vision insurance since 1/1/18. I had been paying taxes on imputed income for their insurance. We were married in October of 2018, and the imputed income stopped at that time. Is there any way to deduct the taxes that were paid on the imputed income on my 2018 tax return now that we are married?
No, sorry. The insurance premiums are properly treated as taxable income for the entire time that you were covering a DP who was not your spouse and not a tax dependent. The fact that you married in October does not change the fact that the premiums were taxable income for January-September.
Beginning after the date of the marriage, the premiums aren't imputed income but that's only if you tell your employer you have a change in coverage. Marriage is a qualifying event, and if you notified them in a timely fashion your premiums for November and December shouldn't be imputed income. (If you are late telling your employer, then you are stuck again, they are doing what you told them to until you tell them something different.)
Since the Janaury-October premiums are treated as being paid with taxable money, you can apply them as itemized deductions for medical expenses. However, due to the higher standard deduction and the 7.5% rule, you may not actually get a tax benefit.