If he doesn’t have mental capacity by then, you cannot apply for LPA, as he won’t have the capacity to give it to you.
Parents do not have to apply for deputyship - significant decisions like medical, placement (in supported living or a care home), social care, etc should all have a best interests meeting with the relevant professionals and you, the family. However, in reality, ime, doctors do not understand the Mental Capacity Act (which is where this idea of best interests meetings is laid down in law), even when there is a poster about it in the room.
It would be better if you applied for deputyship for health and welfare, which makes your legal position clear and gives you the right to be consulted in decisions. (Social Services are another body, who could cut you out of decisions). There is also deputyship for property and finance; but if his only assets and income are benefits, you don’t need deputyship. You can ask the DWP to make you his appointee, which would enable you to apply for and handle his benefits. Appointeeship is enough.
Parents can get the forms to apply for deputyship for the Court of Protection. It’s a court for people without mental capacity. One of the forms is about mental capacity - you would have to find a suitable professional to say DC doesn’t have the capacity to make complex decisions about their welfare - to comprehend, process, retain and communicate back complex information. For instance, if a doctor explained an operation and the risks involved, could DC communicate back 5 minutes later what the doctor had said, and their decision on it? It can be people like a social worker, psychologist, mental health professional, a doctor, etc.
When you have filled in your forms, and got the form about capacity completed, you submit all the forms to the Court of Protection with their fee. I can’t remember how much, but in the region of £400? There’s quite a wait, to see if the Court appoints you as deputy.
Parents can do it themselves; or some get a solicitor to help them.
There are rules about what a deputy can do - such as always acting in DC’s best interests; but some decisions have to go to the Court of Protection.
The Court of Protection also makes the final decision, where there is a dispute say between you and Social Services about what is the best placement for DC. Social Services might go for the cheapest, but you might feel it can’t meet the needs; but somewhere more expensive could. You don’t need a solicitor or barrister to go to the Court of Protection - you can take the case yourself. You present your evidence in a witness statement and a summary of your views in a position statement. It’s best if they are clear, logical and well evidenced, but you don’t have to be a lawyer. The Court wouldn’t look for another placement for you - they expect the parties to present the judge with the choices. It’s meant to be a collaborative process. The Court is available 24/7 in the event of emergencies, such as medical disputes, etc.