In my experience donk bets are frequently a medium strength hand. The sizing was big as noted previously, which is concerning but a good hand is not scared of this board and should be checking. So this person could have a bad ace, 76, 65, 43, 32, 66, 88, and even 99 or TT.
I think I would have preferred a small raise on the flop and real strength will show us at that point. Otherwise you are getting a fold, or a call and likely check on the turn. As played, you don't have as good clarity.
In this case, you are pot committed on a call so could shove, but if he's bluffing you want him to put the rest of his stack in bad. If you stuff it he could fold worse and it will all go in on the river anyway (your stack not his).
Note: edited for clarity