Star-Lord's current power level is unclear, but if he can tap into some of that energy he displayed in Guardians Of The Galaxy and its sequel, he stands a chance at dealing Thanos some decent damage.
Peter Quill is half-human, half-Celestial, and its the Celestial part of him that grants him enormous power. However, at the end of Guardians Vol. 2, Peter was forced to kill his father, and in doing so, he apparently caused his own powers to fade away.
But, even though Peter's daddy was the one who unlocked most of his power and showed him how to use it, it's still implied that he can channel his Celestial abilities using some other source - just like he did with the Power Stone in Guardians Of The Galaxy, which was before he met his father.
So if Peter only has a small amount of the power he displayed in the Guardians movies, he could withstand some of Thanos' attacks. He might even be able to construct weapons like we've seen him do before, and he could also fly through the air and perform huge leaps (even without his rocket boots).
Unfortunately, Peter is also half-human so if his Celestial powers are absent, or diminished... things won't look too good for him. He does carry two space-pistols so he doesn't need to get close to Thanos, but one swift punch could end him if he did.