Yes, it could be a prop. Esp if he settles really easily once he's got it back - he may be reliant on it to sleep. So you would either need to accept that you will need to keep putting it back in, or get rid of it. I am sure neither sound very nice options.... have you looked on the props board for some advice on dummy weaning? You can then decide what, if anything, you want to do
Also I do agree with Lu that his routine could need tweaking. At 8 months you would expect to have dropped that catnap, and it could be that he is now getting too much daytime sleep. What do you think about working towards the routine that Lu suggested?
Bryony