Yes, you can still proceed if he doesn't sign. He will simply have to be
served personally. This can usually be done via Court Bailiffs and will
slow things down a little but not stop them.
If you want to try
to move things forward more quickly you could instruct you solicitor to
write to his saying that if he coopertes fully with the divorce and
returnsthe acknowledgment within 7 days that you will agree to limit your
claim for osts to (say) 50%. That gives him a finacial incentive to
cooperate.
On an UB
petition costs will usually be awrded if sought, unles
the respondet has given good reasons why no costs order should be made.