I thought it was mental trauma from his wife's death, not a physical affliction.
From the opening scene in the Triskelion, I got the impression that:
- He'd already given up using the Ant-Man suit
- His wife had died recently
I think the two events are related.
We know that Jane's death hit him hard – he disappeared for a number of days, became distant from Hope, and lied to her about the death for years. He probably feels a lot of guilt over it, both because it was his suit that killed her, and survivor's guilt because it was meant to be his sacrifice.
He seems to have some idea of what the quantum realm involves – presumably some research after her death – and he didn't visit himself (consider his surprise that Scott returned). But donning the suit would always remind him of his days as the Ant-Man, and how his heroics killed his wife. The guilt and trauma might make him unable to function while wearing the suit.
It's possible that the trauma has been exacerbated by Pym particles. Although the helmet is supposed to protect the brain, it's not totally effective – Hank Pym suffers mental instabilities in the comics despite wearing it (see Michael Edenfield's answer to Do Pym Particles do this in the comics?). The Pym particles may intensify the pain of his wife's death, making it that much harder to wear the suit.