I have up-voted some of the above answers suggesting that forcing anyone into anything is a bad idea, but thought it worth adding my own answer regardless just for completeness.
Avoiding the situation
Assuming you want an in game answer the best method I can think of is to adjust your GM'ing so that this player doesn't actually die. This is too late for now but if you want to avoid my next piece of advise (Which aligns with everyone else) then you can use this method to prevent the situation happening again.
Let the enemy knock him out and take him prisoner or something, have a divine being save his life, let the last sword blow hit the iphone in his pocket and save his life etc.
If this is a problem which you are determined to handle in game you could go all computer-gamey and given them some sort of hook which keeps them alive no matter what - something that maybe teleport's them to a safe place with a portal back to the area once combat ends there. This may change the dynamic of the game as people will take more risks, but that is the balance you will have to work out.
Dealing with the situation [properly]
The use of the word properly is because I (And others it seems) disagree with the premise that you should be looking to force anyone into anything.
You have arrived at the conclusion that you think this player is going to effectively break your game by constantly creating new characters, but (Although you know this individual better than I do) it seems you are not giving him the benefit of the doubt: He may just not like his Ranger.
Even outside of that, if he does want to keep swapping characters, then forcing something isn't the way to handle it.
Talk to your player
Step 1: Ask him why, outside of the game.
Be aware that there is no 'wrong' answer. Whatever his reason if you want him to stay in the game you will have to come to an agreement with him on how to proceed.
Step 2: Come to an agreement.
This is the hard part to answer. If he doesn't like the Ranger and knows what he wants then we are in bonus territory and the fix is just to let him create the new character. If he thinks he struggles to buy into long sessions, or wants to keep swapping characters then you are in a compromise situation (Assuming of course you want to keep him in the game).
I will do a search later on to link some articles, but I have seen plenty of questions on here which will help you reach a consensus on how to proceed. Basically one of both of you will have to adjust and try to find a way that you can both proceed. At this point it may be worth involving the entire group, often the first person to express their feelings is not necessarily the only one that feels that way.
Step 3: Deal with the RP aspect.
It looks like an RP reason is particularly important to you, but once the player has abandoned a character you are within your rights (And the player really should not complain here) to have him become an NPC and you can work out your own RP reason. He might enjoy the afterlife, he might see the bigger picture and find his death furthers a plan which is beneficial to his father or whatever other reasons he has to stay alive. You could even bring him back to life as a proper NPC, though that will need more agreement from the player(s).
Step 4: Profit???