How can i beat this guy in bjj?

i roll with this guy just about every time i go to train but im only 5'9.5" 175 lbs and hes about 6'5+ and 240lbs and he always gets me in submission holds because of his weight, strength, and size advantage. i was wanting to know how i can use his size against him and get him in a kimura or an armbar or a triangle choke without him just straight up muscling out. ( i cant even get this guy in a guillotine because i cant reach my other hand to lock it in). what can i do to beat him?