My nephew is a legal alien attending college in the US. Can i claim him as a dependant with a TIN

It depends -

If your nephew is on a visa type (like F-1) where he is exempt from the Substantial Presence Test  and he is considered a Nonresident Alien, then you will not be able to claim him as a dependent.

However, if he is considered a US resident alien, then if you have an ITIN, you may be able to claim him as a dependent (as a qualifying relative).

According to the IRS:

• You can’t claim any dependents if you (or your spouse, if filing jointly) could be claimed as a dependent by another taxpayer. 
• You can’t claim a married person who files a joint return as a dependent unless that joint return is filed only to claim a refund of withheld income tax or estimated tax paid. 
• You can’t claim a person as a dependent unless that person is a U.S. citizen, U.S. resident alien, U.S. national, or a resident of Canada or Mexico.
• You can’t claim a person as a dependent unless that person is your qualifying child or qualifying relative. 

An individual must meet all 4 of these requirements in order to be considered your Qualifying Relative:

1. Not a Qualifying Child: The individual cannot be your Qualifying Child and cannot be someone else's Qualifying Child. They are a Qualifying Child if they meet all the requirements, whether or not they are claimed as a dependent
2. Relationship: The person must either have lived with you for the entire year as a member of the household (a person who is not actually related to you may meet the requirements in this way), or be related to you in one of the following ways: your child, stepchild, grandchild or other descendant of one of your children (or stepchildren or foster children), son-in-law, daughter-in-law, brother, sister, half brother, half sister, stepbrother, stepsister, brother-in-law, sister-in-law, parent, stepfather, stepmother, father-in-law, mother-in-law, grandparent, and, if related by blood, aunt, uncle, niece, or nephew. Remember that a child whom you legally adopted is always considered to be your child. Also note that, for the purposes of this requirement, divorce or death does not change any relationship which was established by marriage (e.g. son-in-law, daughter-in-law, etc.)
3. Gross Income: The person must have made less than $4,050 in gross income during 2017.
4. Support: You must have provided more than half of the individual's total support during the year

Click this link for more additional information about claiming a dependent