Penis slides out

I never had a problem riding my ex bf. But my bf now is chubby (270 lbs) and obviously has a gut and thickkkkk things. It takes more work for me to ride him n I dont mind cuz I love being on top but I cant do different stances on top without his penis constantly slidding out. I try keep lower but seems like most I can do is grid him. Also reversed cowgirl is impossible for us n I love that position too!! Can anyone relate or give me advice? Is it because his penis is smaller??