Deimodius: Weight counts towards your ability to fly.
Grappling & Flying: Strictly by the rules, you can't fly (other than hover) while grappling, unless you make a grapple check to move, or have Improved Grab and are using only one limb. Describe it as "using eyestalks as handlebars". Certainly, you can prevent movement without actually grappling the motive limb, just by shifting weight and angles. However, in the case of movement which does not require limbs, I'd have the default be "able to move", and the attacker be able to make a grapple check to prevent movement (i.e., a pin).
Regardless, though, the encumbrance factor would have killed the beholder whether he can fly in a grapple or not - he can't fly with a heavy weight on him.