If i claim my boyfriend on my return because i supported him but he also is filing his taxes will he get less on his return because i added him as a dependent?

How much did he make last year?

If your boyfriend earned more than $4,150 you cannot claim him as a dependent. 

If he made less than that and is over 25 years old, it may cause him to get a little bit less back as he would not be eligible for the Earned Income Credit. If he is under 25, he would not be eligible for the credit so his refund would not be affected if you claim him.