It would be best to go back, explain to them what happened and they could determine if further follow up tests are needed and what they find.
Also ask them for a referral to an outpatient neurologist, once they address this concern, who can follow up with this on a regular basis.
To add to the comment above, instead of abusing the ER and costing more money, call his Primary PCP if he is still acting this way now. He/She should have a copy of his ER note anyway or get one. Explain what is going on and your father's symptoms should make him a priority for the PCP.
Sometimes it takes time to be referred to a specialist (Neurologist), but in the meantime, tests can be done. Your father could have a head injury and needs further evaluation by a medical professional. Let us know how things go.