I always liked him as Havok, so nice to have him back!
Let's speculate. I'm guessing it'll be one of these scenario's:
1. He'll just have a big cameo like in DOFP. Maybe we'll see how Cyclops gets recruited and Havok is part of that moment, since he's family and maybe they're living together (father, uncle, cousin,...). He could motivate Cyclops to go to Charles to learn how to control his powers, like he did in the 60s. It wouldn't be too hard having him only in 1 or 2 scenes to deal with the age difference. With the right camera angle and make-up he could look older.
2. Right after DOFP he gets somehow recruited by Apocalypse, giving him new technology that slows the aging process and making him one of his horsemen. He could fight Cyclops in the final battle. I don't think the horsemen will have that much development or dialogue, aside from maybe Angel. It seems improbable that Havok will have a role bigger than in FC, so it could be a decent role for a side character like Havok.
On the other hand, this time Till got announced much earlier, so maybe he does have a bigger role than in DOFP.