I wouldn't say that's a mental capacity issue, tbh. It sounds like he was happy to make the decision to take the medicines (which is the capacity bit) but found the execution of his decision more challenging.
Capacity has 4 elements:
Understand
Retain
Weigh/decide
Communicate
So if he can understand that his medicine is changing
Retain the information long enough to make a decision
Weigh up the options (to take it or not)
and
Communicate his decision to you (even if just a nod of agreement)
Then he has the capacity to make the decision.
If he then, having made the decision, can't remember what he was meant to do, or to go and get the medication, etc., you can help him with that.
Remember too, that steps such as breaking down information, giving pictorial information, giving information at a more appropriate time, or breaking the decision down into a series of decisions, can all be ways of helping someone make decisions that may at first seem beyond them.
An example from our situation, which might help, although more of a property and finance decision:
DD1 has mixed capacity. She has new carers coming and she doesn't like using the lounge to meet with them. We thought a log cabin in the garden might be a good use of her money. But, they are expensive, and variable in quality.
Understand - DD1 understands that a cabin would be good. She doesn't understand that some companies are passing off poor quality cabins as if they are good. She understands that you can buy cabins new or second hand, but she doesn't understand that an ex-display cabin might be better quality than a new one.
Retain - DD1 has variable retention. She retains the minor details but not the 'big picture'.
Weigh - DD1 finds weighing information very difficult and is very impulsive. She just wanted to go home because the road was loud.
Communicate - DD1 communicates that every idea is brilliant as long as we get it done.
Overall, we decided that she didn't have the capacity to decide what to buy, so we made the decision for her, but let her choose the paint colour.