Welcome to HCM Forum,
Considering he is patient of Bipolar disorder
nearly his life and current age and delay in start of treatment your father might need treatment for lifelong,Though dose of drug and number of drugs may vary over period.
This is now a kind of illness like Diabetes
or Blood pressure where patient is only improved on medication and not completely cured and he needs daily regular medication with changes according to change in illness ,general physical condition.
There are en number of listed side effects of each an every drug but very few are seen clinically and fewest of them are to be important to be concerned.As rightly u know sedation is one of them( which deceases over a period),other one weight gain which not seen in your father.Medicines are determined according to need of patient if sleep disturbances present then sedative medicine used,low weight present then appetite rising drugs used and so on.There are chances that unexpected side effects might occur ( though rare approx 1/100000)
So its always advisable to have continuous and regular follow up with your psychiatrist
to have answers for all questions arises over period and to convey what problem he is having so change treatment accordingly.
Hope this will helps.