Khyan is a bit older than my DS and has the same amount of sleep so I wouldn't have thought that it was too much,.
We also have a timer but DS doesn't come in, he shouts out. So this morning, he shouted 'Mummy, I'm awake' and I shouted back that it was the middle of the night and he should go back to sleep. He shouted back 'I'm just checking' ANyway, I think he dozes a bit but at 6.15 when the light came on he shouted so he must have been awake then.
Now, whether Johno is a lazy devil I don't know but he waits for me to go and get him! However, when he went into a big boy bed, I put a stair gate across his door so that he was safe in the night. Maybe try this, maybe my LO has worked out its more comfortable to lie in bed than stand at the door, I don't know but I don't think you are asking too much.
Also, I think if he does get up, you just have to keep taking him back and repeating the rule which is not to get up til the light comes on. You could put some toys or books at the end of the bed for him to busy himself with if he wakes early