Sleeping in the day

Should I be putting my 7 week old down for a nap at a particular time? At the moment I let him nap when he is tired, which (if we are at home) is about 15 minutes in the morning, about half an hour to an hour after lunch/early afternoon, and then about two hours late afternoon/early evening. All these naps are on a cushion or in his bouncy chair. If we go out in the car or pram he sleeps then too. My concern is that his longest sleeps are early evening - which means I can't get him to sleep for bed until about 10pm. But he isn't tired earlier in the day - so how am I supposed to make him sleep?!