Well I just went thru this process, so depending on how long the relationship has been, they will need to build there own (BOND), in my situation we dated for 1yr before I let them hang out,but before that he would be mean to him, hit him, my BF could not touch me at all, he would move his hand, if we were sitting on the couch he would just move right in the middle of us. So it took some time, I would have him read to him, they would take the trash out together, my BF bought him a Motorized Car, which he loves, that they take out in the yard and he play's with till it dies, then we had mommy time, and my Bf would just watch, he would not touch me, or interfear, then over more time he would join things we did, no my son is still very clingy, and Im his, but he shares me now, but it was a struggle, but we will soon be Married, and he address him as Dad now. And the more time they spent together the better the relationship with me not around, and with out any dicipline on his part, but ur son has been the man of the house, and now he has this new person invading his space and taking his mom, so yea hes gonna be a lil over protective, and dicipling him for actions for loving his mother and alil selfish, No I dont think so, he needs to understand that ur not chooseing anyone over him. And having respect for your BF must be est, but thru time, and it will build as they bond.
Hope This was Helpful