70 % of 7 months = 4.9 months; round to 5. 5 months together + 5 months with you = 10 months. Answer 10 months.
For Dad, it's 5 + 30% of 7 = 7 months. Since you had the child for the required "more than half the year", either one of you can claim the child this year. But, if you cannot agree on who that will be, you are the one with the right to claim the child, because you had the child the longest (the tie breaker rule). For tax purposes, you are the custodial parent. This essentially means you get to decide how the child will be filed.
Assuming the 7 months was in the last part of the year (Jun - Dec), the "special rule" is available for you and dad to use.
There is a special rule in the case of divorced & separated (including never married) parents. When the non-custodial parent is claiming the child as a dependent/exemption/child tax credit; the custodial parent is still allowed to claim the same child for Earned Income Credit, Head of Household filing status, and day care credit. This "splitting of the child" is not available to parents who lived together at any time during the last 6 months of the year; then only one of you can claim the child for any tax reasons. The tax benefits may not be split in any other manner.
Note in particular that the non-custodial parent can never claim the Earned Income Credit, Head of Household filing status or the day care credit, based on that child, even when the custodial parent has released the dependency to him.
So, it's good idea to let the other parent know that you will be claiming those items, as many first time divorced parents are not aware of this rule and may try to claim those items, which will cause the IRS to send out letters.
Ref: https://www.irs.gov/publications/p17#en_US_2017_publink1000170897
Scroll down to "Children of divorced or separated parents (or parents who live apart)"
