
I think eighteen months is probably the worst time to own a male Dalmatian. They become a right Jack the Lad. If mine is anything to go by, at this age he was very similar on the lead but fine when let off (unless he met up with an entire male). He has grown out of it now, still a little iffy with entire males, but no problem with other dogs in general. A Dally will always be territorial to a certain extent.
You are probably making things worse without realising it by tensing yourself and tightening the lead when another dog approaches. Try to relax and let the lead go loose and avoid a front on confrontation with him, cross the road if necessary. I found using a Halti during this time very helpful as you then have control of the head and can turn it towards you.
Hope this helps
Sandra