DS (3.5) recently terrified of certain things, dont know why- any ideas?

[vnmum]

my DS has over the past 2 weeks suddenly become terrified of the dog or a friend going near rivers, streams etc when out for a walk. i have been looking after my parents dog for a few weeks and have been taking him on a walk by a river. the path isnt right next to the river all the time and its safe. if the dog is let off the lead, ds just seems terrified, crying asking him to be put back on the lead, quite distressed. he's the same if his friend who sometimes comes with us with her mum walks too far in front or near the edge of the path. he wont leave my side and wants to hold tghe pushchair all the time instead of running and playing with his friend. part of the walk involves crossing the river over a bridge. DS is fine with this and will look over the barrier at the river , throw sticks in etc but if his friend goes near the barrier as we are crossing he starts to panic.

i have tried reassurance but it doesnt make any difference. i just dont know why he seems so scared. even my friend was shocked at his reaction. has anyone any insight into this or any ideas as to what i can do.

my DH went away in march for work and isnt back till august so im not sure if its got anything to do with that but i thought he was dealing fine with it as ive explained where DH is using a globe and he says where he is, he happily talks to him on phone etc