Had this with ds. Tried suppository - worked once - not second time. Lactulose and senna had no effect.
Got given lots of advice on upping the amount of fruit he eats, giving him a whole-grain cereal for breakfast, wholemeal bread, brown rice and fruit juices etc.
They do say with constipation that you should keep giving them the drugs for quite a while until they forget that it's painful to do a poo. (Ds would hold on and on to it compounding his problems.) It might be that's all you simply need to do.
I also got hold of a mix of chamomile and fennel tea from the health food shop(if it looks like it's sprouting that's the one you want - you serve it cold obviously) to give him mixed with apple juice (which, again, is particularly good for the insides). It's meant to help get your insides moving. He'd have this over the course of the day. Spirulina (you need advice on getting the right type) may also help (natural bulking agent). It tastes quite chalky so you need to mix it with yoghurt or something like semolina.
Also you can try and up the amount of exercise they do and if you can get them to have a hot bath (as hot as they can stand) that helps too. Also - if they can take it an abdominal massage with lavender or chamomile oil (relaxant) - ds couldn't bear having his stomach touched though because he was so bunged up.
In reality my ds figured out what made his bowels move and resisted all those things because it hurt him so much making the cycle even more vicious- but you may experience something different.
In the end I took him to an osteopath (was desperate since he'd been in such pain since it had all started 5-6 week previously). Had a (admittedly painful for him) poo in the waiting room immediately after, and a good old clear-out (sorry if tmi!) over the next few days and has been regular since. Gets bunged up when he's ill, but another trip sorts him out again.
Also got shown some balancing exercies to engage his stomach muscles which has helped. A friend got advised to hold her son in a sitting position when she thought a poo was coming, becuase it's much more difficult for them to resist doing a poo in that position, but my ds (2.5 - hers 1.5) is too strong for me to do this.