5 months - cold turkey switch to formula??

[Imeg]

I have been breastfeeding my 5 month old who is doing really well but I have been ill for the last 4 weeks. Although it's nothing serious (doctor says either IBS flare up or virus) it is making me feel really grotty in combination with breastfeeding because I'm not eating properly. Although I would like to have the option to continue bf I am now sick of feeling ill and my priority is to get better so I have the energy to look after the baby properly.

So I would really like to swap to mostly formula, ideally with one breastfeed a day to try to maintain supply and then perhaps drop the formula and up breastfeeds once he's on more solids and needs fewer feeds and I'm feeling better.
I think the only way he will take this is to completely stop breastfeeding for at least a couple of days as he's currently refusing bottles - we have been trying one formula feed a day but it's not going well, probably because he's not hungry enough.
I am worried about mastitis but my plan would be to express enough to make myself comfortable (only have a hand pump but hopefully it would be enough if I'm really full) - any other tips or is it really inadvisable to stop suddenly at this point? Then after a few days I would plan to reintroduce one feed a day - would this be enough to keep my options open for the future? If not, as I say my priority is to get better so it's not a disaster if my supply dries up.

Do what you will - it all sounds sensible! You have already done well by your baby! It ought to be law that every baby gets his or her colostrum (regarded as so important in farm animals that farmers move heaven and earth to make sure a lamb/calf/foal etc gets it). It is the primer for mammal babies and to dismiss it as the watery first milk has been a scandal for decades.
Bear in mind that every baby has preferences though; my DS adored my milk (human milk is desperately sweet as you will probably know if you have tried it!) and when in day nursery they ran out of my milk and offered him some artificial baby milk (NOT Formula please as that is marketing speak!) He took one taste of the flat tasteless stuff and look at the nursery nurse as if she was trying to poison him! DD on the other hand found the bottle at nursery so much less effort than me and suddenly gave up aged 7 months - leaving me to get mastitis as we were going away over Christmas and I had left my pump at home...
Please DO worry about mastitis as it is not nice! Btw I weaned DS aged 9 months - slightly later than planned as he was poorly and needed the comfort. Then onto follow-on milk.

Only you can manage it for you, your body and your baby as we are all different. All the best!

Thank you, yes mastitis does sound nasty and the last thing I need is to feel worse! I have no qualms about stopping breastfeeding if that's the best option for us all but I'm just not sure about the best way to go about it.

(NB This is a side issue but since it's been mentioned I would like to point out that there are important species differences in the structure of the placenta so that calves, lambs and foals don't get antibodies across the placenta in the same way human babies do. Therefore the colostrum is much more important for them than it is for human babies, whatever the benefits for human babies may be.)