I sort of touched on this in another thread but it's probably more suited here. I understand that there is no right or wrong way to do things and that you can't necessarily tell me what I should or shouldn't do but I'm at my whits end and could really do with some guidance.
My DS 11 months has never been good with sleeping at night. I think he's 'slept through' twice (from 11pm until 5am so not really sleeping through). He generally goes down between 7pm and 9pm depending on when his last nap was and then wakes every 1-3 hours and will take a bottle every time, even if it's just 2oz. He normally wakes for the day at about 5 or 6am.
Through the day he used to manage 2-3 hours between each nap which worked out great because I could plan our days accordingly but now he tends to only have two 2 hour naps which he'll take whenever he feels like it; it can sometimes take me an hour to get him to go to sleep even though he is obviously exhausted and ratty. If he doesn't take his second nap until later he naturally won't be ready for bed until about 9pm which isn't ideal at all.
Now that I am back at work our weeks can be quite erratic. He spends Mon, Tues & Thurs with my DM and for 2 hours on these days with a childminder while I and DM are at work. On a Tuesday MIL has him from 4pm until 8pm then on a Thursday we go to DM's house for tea and get home around 8pm, both days he falls asleep in the car and we put him straight to bed when we get home.
He hardly ever naps when he's with DM (or anyone else for that matter) and if he does it's only for about 30 minutes in the morning. Because she is quite busy when she has him it would be difficult for her to follow through with any kind of routine I implement. When he's with DM his day normally goes as follows
8:30am school run
11:30am - 1:30pm childminder while DM goes to work
3:15pm - 4pm school run for my DS and DM often picks my other DS up from work
He doesn't have any kind of routine at all (day or night), we've always gone with the flow 
Should I start and "force" naps at certain times through the day?
Should I start waking him after about an hour during the second nap?
Can I really implement a routine when it's only realistically going to be in place 4 out of 7 days?
Any other guidance and advise would be warmly welcomed!