2 year old herman tortoise

hello i brought this as a christmas pressie,he seems to be happy he is eating and we let him out daily we have got the rightn tank for him as told by the reptile shop but he keeps trying to climb all the time and is falling on his back?what can i do to stop this also how long would he last on his back? im worried incase he does it when im not there.if there4s any other tips it would be much apprieacted thanks