SOAD
Wise Old Thumper
Where's your nearest RSPCA? Try giving them a ring and ask if they are doing the neutering scheme. There's different policies in each branch, but many will send you a voucher even if you don't get benefits.
Not sure how much neutering costs in your area- but if it's quite reasonable like my vets at less than £35 you could be paying only £20 with a voucher (usually gives you around £15 off).
I'm afraid I do think it is your responsibilty to get Zazu neutered, whether you end up keeping him or not. If the bonding doesn't work then perhaps you could strike a deal with a rescue that you will swop him for a female to go with Kovu. Although they may not agree with the space you have...
I think that is Lucyhp, Lucy?