It doesn't matter if he claims not to have received it.
As long as you send it to his last known address then the claim has been served on him.
Once you have done this, if he does not respond in time then you can claim judgement in default. In other words, you win automatically.
At this point the other side may well apply to have the judgement set aside and be relisted, claiming that they never received the claim form. All this means is that they can no longer just bury their head in the sand and have got to deal with the issue.
However, a lot of people don't do this - largely as they are unaware that they can - and just pay up at this stage.
One thing to be aware of with county court claims, if the debtor doesn't pay off his own back then it's up to you to actually get your money.
There are a number of things you can do. One possible thing to think about, if the total amount owing is at least £600, is to get a High Court Enforcement Officer involved. These aren't bailiffs, they actually have a lot more powers than bailiffs.