Collin loves his grandpa!!

Me

My son loves his grandpa! I'm a single mom and my dad has been the only male in his life that has constantly been there for him. We see him 3-4 times a week and Collin gets so excited, gives him a million hugs and will sometimes call him dada! But recently everytime my dad leaves he throws fits. Sometimes we'll FaceTime time and the last couple of times Collin saw him he started crying and ran off. I know he misses him and wants him around more but I feel so bad there is no way to explain anything to him. Also my mom and I (who we are currently living with) don't always get along and I know he doesn't like tension. I feel like a bad mom sometimes for it but it's really my only option for at least 5-6 more months until I can get my own place. Is there a way to make things easier for my son? I'm at a loss