If the boyfriend is going to be contributing to the mortgage you'll need to make sure that both he and you regard it as rent so that he doesn't have a claim on the house should you split up.
There have been a gazillion threads on this before, there have also been a gazillion threads about parents going guarantor for their kids and instead of having a happy and fulfilling retirement are having to sell their houses to bail out the child they went guarantor for, if you value your parents you will not allow them go guarantor on your mortgage.
Such a messy situation, it's not easy nowadays ;-(