<apex:pageBlock title="Select the services that are applicable to this milestone.">
        <apex:pageblockSection >
            <apex:selectCheckboxes value="{!selections}">
                <apex:selectOptions value="{!ServicesNameToID}" />
            </apex:selectCheckboxes>
        </apex:pageblockSection>
    </apex:pageBlock>


    <apex:commandButton action="{!createRelatedServices}"
                        value="Add Related Service" id="button"/>
</apex:form>

public Map<String, ID> ServicesNameToID {get;set;}
public pse__Milestone__C milestone {get;set;}
public pse__Milestone__C queriedMilestone {get;set;}
public String[] selections {get;set;}

public RelatedServiceController(ApexPages.StandardController stdController) {
    milestone = (pse__Milestone__C)stdController.getRecord();
    getServices();
}

public void getServices(){
    //Need to fully query the milestone to get project ID/opp ID
    queriedMilestone = [SELECT Name, ID, pse__Project__c, pse__Project__r.pse__Opportunity__c from pse__milestone__C where id = :milestone.ID];

    List<OpportunityLineItem> oppLineItems = [SELECT name, ID, Product2ID, ProductName__c from OpportunityLineItem
                                              WHERE OpportunityID = :queriedMilestone.pse__project__r.pse__opportunity__c];

    ServicesNameToID = new Map<String, ID>();
    for(OpportunityLineItem oli: oppLineItems){
        ServicesNameToID.put(oli.ProductName__c, oli.Product2ID);
        //system.debug(oli.ProductName__c);
        //system.debug(oli.Product2ID);
    }

}

public PageReference createRelatedServices(){

    List<Related_Service__c> relatedServicesToInsert = new List<Related_Service__c>();

    if(selections != null && selections.size() > 0){
        //Strings of the service name will be returned - match them up with the ID of that service using our map
        for (integer i = 0; i < selections.size(); i++){
            ID serviceToRelateID = ServicesNameToID.get(selections[i]);

            Related_Service__c rs = new Related_Service__c();
            rs.Milestone__c = milestone.ID;
            rs.Service__c = serviceToRelateID;
            rs.Name = 'test';
            relatedServicesToInsert.add(rs);
        }
    }

    insert relatedServicesToInsert;
    PageReference pageRef = new PageReference('/' + milestone.ID);
    pageRef.setRedirect(true);
    return pageRef;
}