That depends on what his attack would be. If it would be a double or single leg takedown, because that is the easiest and fastest way for him to take you down, just go to your knees and and then either move outwards or just attack his head with punches and elbows or even a choke if you can get a hold of his neck (Gi or noGi or whatever). Even if he immediately gets you to the ground don't let him get on top of you, always get out of the line of attack be it standing or on the ground meaning move sideways or try to get immediately on your knees.
Look at how Kazushi Sakuraba (Gracie Hunter) reacted to the leg attacks of the Gracies, he always tried to maintain a good posture and he never went into a grappling game with them, he played his game not theirs:
You can even see the usefulness that Suwari Waza or Shikko movement have in that situation, although Sakuraba doesn't do Aikido , in the way he keeps his balance while on the knees. I actually couldn't get into seiza because the stiffness in my knees but after Shikko movement and Suwari Waza it wasn't a problem anymore.
If he tries to clinch for a Judo type throw all you have to do is keep your hips low and push his hips away. Also knees to the groin in that situation will keep him away. Even in the MMA world guys can't take groin attacks easily or not at all, especially if they come in multiples: