Put the baby in a sling if your DS will still need to use the buggy. Although if he is only just 2 you will probably still get some mileage out of a double (have no reccomendations though). If he is escaping the buggy harness then try one of those walking harnesses - you should be able to clip it on the D rings of your buggy (hopefully has some?). It should be tighter, therefore harder for him to remove. Advantage also being if he needs to walk you can just clip on the reins to keep him close (if he is a bolter!)

If you're going to get a new pram - given your current situation - I'd go for what I'm getting - a Mutsy - and get the "fun seat" attachment, use it right now - while your preg and you want to keep your ds IN the pram - then get the double seat and click it in place when you want two seats after baby arrives!

My DS (nearly 2y) escaped his baby jogger last week and his 2y old pal managed to escape her maclaren too - so it is a common problem with wriggly toddlers!

I was looking for a parent facing pushchair - and in my searching and thinking came across the mutsy 4rider.
It can take a tandem seat (not that I need this bit!) and also a fun seat (they click in and out - so the fun seat could be used while baby is in the sling) - and for a really good sling that you'd be able to carry a newborn up to toddler comfortably in - look at big mama slings- she does some really nice soft structured slings like the ergo that can be used from birth - and they're really fashionable too!)
Sorry this all sounds expensive - but there are options.

The other less expensive thing to do - is to look in the base of your buggy/pram - are there any "D" rings (I think they all have them normally - even if there on the frame of the pram with slits into the seat to access them) - get a harness - like the highchair/walking ones clippasafe harness reins kit put the short tethers around the d rings - snuggly adjust the harness about your ds - and clip into the buggy - by all means use his pushchair harness also to stop him shoving his bum off the seat and hanging there as he tries to escape!

If you are going to get a double - the mutsy 4rider with a tandem seat unit might be a good versatile option - and the fun seat is around the price of a trike - (looks like one!)...but there have been some great deals on eBay recently too - so look on ebay if you don't want to spend so much.
mutsy prams

No - I hadn't really heard of Mutsy either - but everyone seems to really rate them - they certainly look good - with loads of really useful accessories too - and having the option to have baby face you is great - wonder if the tandem seat can face you? the Oxford pram center seems to have really good deals and they claim all their stuff is in stock - so you can always phone them to chat (I did and they were fab - very informative!).

Like I said - some really great 2nd hand deals on eBay too.

I've been a bit of a pramaholic slight understatement over the years (got 4 kids in my defence)! For me the Mutsy 4 Rider Light seems to tick all the boxes (so much so that I'm selling my "fleet" of prams and hoping this one "does all"! (everyday, offroad, public transport, shopping, holidays).

I love slings too, although I've had really bad back problems on and off over the last 2 - 3 years (suspected slipped disc) so I do recognise that I have to mostly use a pram these days. Wrap slings are the most comfortable for me - when I am using them.
Definately get your friend to hand it back before your baby arrives if you can!