You can back carry from 6 months, in theory! I got on great with a Mei Tai, which can be used on your front and then when you are ready on your back. Or you could use a SCC Soft structured carrier, which is a mei tai with buckles. Lots of info can be found on www.slingguide.co.uk and a dedicated forum at www.naturalmamas.co.uk
I have a Mei Tei and a Beko - which is the same thing but with rucksack-type straps. MT gives a better fit but I only use it indoors as I don't want the long straps trailing through muck while I tie it - particularly on crap weather days. You can use both from baby through to age 2-3.
Thanks for your suggestions. Had a look at mei tai and it looks nice, think only issue is that it is quite limiting of what he will be able to see. He loved being forward facing in old sling. But guess as it's only short journeys it doesn't matter.
I've looked on ebay and may get one off there but I'll have a look at the ergo first.
I have a ring sling that I use daily with my 12 month old (not sure how much he weighs) - perfect for nipping to the shops, walking the 10 minutes to nursery etc. He can sleep in it too on my front but to be honest after about half an hour it does make my shoulder ache.
For anything longer than a short trip I use a Beco Butterfly - you can have it on your front, but I mostly have it on my back now he is a bit heavier. He can sleep on my back too. It's great because you can put the baby in it first and then put it on your back (or go from your front to back) so not as fiddly as a lot of back carriers.
Sorry if this is too late OP but I've just received my Baby Bjorn Comfort this morning and thought I'd recommend to you. My DS is 28 weeks and almost 20lbs, and just about grown out of his standard Baby Bjorn. It also killed my shoulders to carry him in it for more than half an hour. I had an Ergo Sport, which was comfier to carry DS but I found it hard to get him in it and I didn't like the way there's no built-in head support so if DS falls asleep I have to fart about pulling the head support out of it's pouch and clipping it on, which sometimes wakes him up. Anyhoo, my new Baby Bjorn Comfort is the bees knees. It works like an Ergo in that it has a big padded hip belt which takes all the weight, but the pouch where you put DC is rigid so you can post them in easily. You can carry them until about 2 y.o facing inwards and until 25lb facing outwards, which you can't do with the Ergo and which DS loves. Just been for a 2 hour trek around East London and I honestly didn't feel a thing in my back or shoulders. Got mine brand new on EBay for £90 so same price as Ergo. Not cheap but blimmin' worth it!
I can't speak highly enough of mu vaude baby light backpack. I actually used it for my rather large boy from four months with great success. I have a border collie who needs lots of exercise and the sling I had wasn't sufficient. It was expensive (£140) but we are still using it daily 11 months on. I have used it to go to the zoo, shopping, you name it. He can look about and loves it.