My ex claimed my son in 2019 even though he lived with me 1/2 the year and I paid for almost everything.im not trying to claim him and fine with this but stuck here?

The parent who the child lived with the most number of nights in the year is the one who is the custodial parent and has the right to claim all the tax benefits for the child. If the child lived with you less than half the number of nights in the year, then you should say the child lived with you 5 months. If the child lived with you more than half the nights of the year, then you are the parent who can rightfully claim the him.

Here are the rules: For divorced or separated parents or parents who live apart, the custodial parent, if eligible, or other eligible person who the child lived with for more than half the year (183 nights in 2019), can claim head of household filing status, the credit for child and dependent care expenses, the exclusion for dependent care benefits, and the earned income credit. The non-custodial parent, if allowed by divorce decree or consent of the custodial parent on form 8332 or similar signed statement, can claim the dependency and child tax credit. For post-2008 divorce decrees or agreements, form 8332 or similar signed statement is required. The child tax benefits cannot be split any other way.