I've been practicing fisting and large anal insertions for over 10 years and the first thing you need to know about it is that it is not an overnight thing. It takes a lot of time and patience and I'm talking over a year!!
If you've never taken anything bigger than a cock up your ass then the first thing you need to do is experiment on your own and learn to stretch your ass gently and gradually EVERY DAY to reach your goal of taking a fist. To do this you'll need some butt plugs that gradually increase in size, train with them till you're comfortable with the largest size and then move up in size. It's like stretching any muscle, to get it as loose as ou want it and not suffer any permanent damage, you have to go SLOW.
Now let's talk about the pain. There are two types of pain you will experience, one of which you DEFINITELY want to stay away from. The first is stretching pain. This is the initial discomfort you will feel when you first stretch your ass past it's limits. Once you feel this pain stop immediately and wait. Don't take the toy out, just wait a bit, take a few deep breaths and see how you feel. Never push super hard. Always apply steady even pressure and breathe deeply till the toy just won't go any further. Sometimes it helps to bear down really hard for 3 seconds and then relax your ass. As soon as you relax it you can feel the toy slide in a bit deeper. If it doesn't don't worry, that's your ass telling you it's going as far as it's gonna got TODAY. Do this for 20 mins per day and you will feel your ass start to open up easily and quicker.
The next type of pain is the pain you definitely don't want. It usually happens when you are feeling really confident that "today" you can do it! Today is the day you will reach that new width you've been trying for but then all of a sudden you feel an unusual sharp pain and the rest of your stretching session is just painful no matter what you try. This is your poor cute little ass telling you to STOP!.... So do it. Stop immediately and give your ass a rest for about 3 days. If you don't listen to your body at this point you'll pick up some bad habits which could and often do lead to permanent damage.
Lastly let's talk about creams and lubes. When it comes to lube it's always about personal opinion. In my 10+ years of experience I've tried all the fancy lubes but for anal sex, it's pretty simple. VASELINE, CRISCO VEGETABLE LARD AND BODY MOISTURIZERS are the best for anal stretching and ass play. Why? Well it's quite simple. Oil based lubes won't thin out, are usually slicker and last far longer than water based lubes. This is what you need when stretching out your ass. Also, if you can get it, I've discovered a product called Albolene which is a makeup remover but as far as anal lube goes it is the king of the slipperies. :)
Oh - and always use tons more lube than you think you need. Lube your ass up and once you think you're using enough ADD MORE!
DESENSITIZING CREAMS: rule of thumb. Don't EVER use them when exploring new widths or depths. If your body is too numb to feel pain how are you gonna know when to stop? Only use them when you're playing in your "safe" zone and set it aside when training for wider toys
In conclusion just remember to listen to your body, go very slowly, use tons of lube and enjoy tge journey. It may take months, it may take years but it's your body so work with it and NEVER let your BF push you into something you're not comfortable with. If he tries anything, offer to do the exact same to him and watch him chance hie tune ;)